abortive initiation
Process during initiation of transcription in which RNA polymerase repeatedly generates and releases short transcripts, from 2 to 6 nucleotides in length, while still bound to the promoter. Occurs in both prokaryotes and eukaryotes.
acceptor arm
The arm in tRNA to which an amino acid attaches.
acentric chromatid
Lacks a centromere; produced when crossing over takes place within a paracentric inversion. The acentric chromatid does not attach to a spindle fiber and does not segregate in meiosis or mitosis; so it is usually lost after one or more rounds of cell division.
acidic activation domain
Commonly found in some transcriptional activator proteins, a domain that contains multiple amino acids with negative charges and stimulates the transcription of certain genes.
acrocentric chromosome
Chromosome in which the centromere is near one end, producing a long arm at one end, and a knob, or satellite, at the other end.
activator
See transcriptional activator protein.
adaptive mutation
Process by which a specific environment induces mutations that enable organisms to adapt to the environment.
addition rule
States that the probability of any of two or more mutually exclusive events occurring is calculated by adding the probabilities of the individual events.
additive genetic variance
Component of the genetic variance that can be attributed to the additive effect of different genotypes.
adenine (A)
Purine base in DNA and RNA.
adenosine-3′,5′-cyclic monophosphate (cAMP)
Modified nucleotide that functions in catabolite repression. Low levels of glucose stimulate high levels of cAMP; cAMP then attaches to CAP, which binds to the promoter of certain operons and stimulates transcription.
adjacent-1 segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-1 segregation takes place when N1 and T2 move toward one pole and T1 and N2 move toward the opposite pole.
adjacent-2 segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-2 segregation takes place when N1 and T1 move toward one pole and T2 and N2 move toward the opposite pole.
A-DNA
Right-handed helical structure of DNA that exists when little water is present.
affinity capture
Use of an antibody to capture one protein from a complex mixture of proteins. The captured protein will “pull down” with it any proteins with which it interacts, which can then be analyzed by mass spectrometry to identify these proteins.
allele
One of two or more alternate forms of a gene.
allelic frequency
Proportion of a particular allele in a population.
allopatric speciation
Speciation that arises when a geographic barrier first splits a population into two groups and blocks the exchange of genes between them.
allopolyploidy
Condition in which the sets of chromosomes of a polyploid individual possessing more than two haploid sets are derived from two or more species.
allosteric protein
Protein that changes its conformation on binding with another molecule.
alternate segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then alternate segregation takes place when N1 and N2 move toward one pole and T1 and T2 move toward the opposite pole.
alternation of generations
Complex life cycle in plants that alternates between the diploid sporophyte stage and the haploid gametophyte stage.
alternative processing pathway
One of several pathways by which a single pre-mRNA can be processed in different ways to produce alternative types of mRNA.
alternative splicing
Process by which a single pre-mRNA can be spliced in more than one way to produce different types of mRNA.
Ames test
Test in which special strains of bacteria are used to evaluate the potential of chemicals to cause cancer.
amino acid
Repeating unit of proteins; consists of an amino group, a carboxyl group, a hydrogen atom, and a variable R group.
aminoacyl (A) site
One of three sites in a ribosome occupied by a tRNA in translation. All charged tRNAs (with the exception of the initiator tRNA) first enter the A site in translation.
aminoacyl-tRNA synthetase
Enzyme that attaches an amino acid to a tRNA. Each aminoacyl-tRNA synthetase is specific for a particular amino acid.
amniocentesis
Procedure used for prenatal genetic testing to obtain a sample of amniotic fluid from a pregnant woman. A long sterile needle is inserted through the abdominal wall into the amniotic sac to obtain the fluid.
amphidiploidy
Type of allopolyploidy in which two different diploid genomes are combined such that every chromosome has one and only one homologous partner and the genome is functionally diploid.
anagenesis
Evolutionary change within a single lineage.
anaphase
Stage of mitosis in which chromatids separate and move toward the spindle poles.
anaphase I
Stage of meiosis I. In anaphase I, homologous chromosomes separate and move toward the spindle poles.
anaphase II
Stage of meiosis II. In anaphase II, chromatids separate and move toward the spindle poles.
aneuploidy
Change from the wild type in the number of chromosomes; most often an increase or decrease of one or two chromosomes.
annotation
Linking the sequence information of a gene that has been identified to other information about the gene’s function and expression, about the protein encoded by the gene, and about similar genes in other species.
Antennapedia complex
Cluster of five homeotic genes in fruit flies that affects the development of the adult fly’s head and anterior thoracic segments.
antibody
Produced by a B cell, a protein that circulates in the blood and other body fluids. An antibody binds to a specific antigen and marks the antigen for destruction by a phagocytic cell.
anticipation
Increasing severity or earlier age of onset of a genetic trait in succeeding generations. For example, symptoms of a genetic disease may become more severe as the trait is passed from generation to generation.
anticodon
Sequence of three nucleotides in transfer RNA that pairs with the corresponding codon in messenger RNA in translation.
antigen
Substance that is recognized by the immune system and elicits an immune response.
antigenic drift
Used in reference to a rapidly evolving virus, with new strains appearing frequently because of mutations.
antigenic shift
Used in reference to a virus that has undergone major changes in its genome through the reassortment of genetic material from two different strains of the virus.
antiparallel
Refers to a characteristic of the DNA double helix in which the two polynucleotide strands run in opposite directions.
antisense RNA
Small RNA molecule that base pairs with a complementary DNA or RNA sequence and affects its functioning.
antiterminator
Protein or DNA sequence that inhibits the termination of transcription.
apoptosis
Programmed cell death, in which a cell degrades its own DNA, the nucleus and cytoplasm shrink, and the cell undergoes phagocytosis by other cells without leakage of its contents.
archaea
One of the three primary divisions of life. Archaea consist of unicellular organisms with prokaryotic cells.
artificial selection
Selection practiced by humans.
attenuation
Type of gene regulation in some bacterial operons, in which transcription is initiated but terminates prematurely before the transcription of the structural genes.
attenuator
Secondary structure that forms in the 5′ untranslated region of some operons and causes the premature termination of transcription.
autoimmune disease
Characterized by an abnormal immune response to a person’s own (self) antigen.
autonomous element
Transposable element that is fully functional and able to transpose on its own.
autonomously replicating sequence (ARS)
DNA sequence that confers the ability to replicate; contains an origin of replication.
autopolyploidy
Condition in which all the sets of chromosomes of a polyploid individual possessing more than two haploid sets are derived from a single species.
autosome
Chromosome that is the same in males and females; nonsex chromosome.
auxotrophic bacterium
A bacterium or fungus that possesses a nutritional mutation that disrupts its ability to synthesize an essential biological molecule; cannot grow on minimal medium but can grow on minimal medium to which has been added the biological molecule that it cannot synthesize.
backcross
Cross between an F1 individual and one of the parental (P) genotypes.
bacterial artificial chromosome (BAC)
Cloning vector used in bacteria that is capable of carrying DNA fragments as large as 500 kb.
bacterial colony
Clump of genetically identical bacteria derived from a single bacterial cell that undergoes repeated rounds of division.
bacteriophage
Virus that infects bacterial cells.
balance hypothesis
Proposes that much of the molecular variation seen in natural populations is maintained by balancing selection that favors genetic variation.
Barr body
Condensed, darkly staining structure that is found in most cells of female placental mammals and is an inactivated X chromosome.
basal transcription apparatus
Complex of transcription factors, RNA polymerase, and other proteins that assemble on the promoter and are capable of initiating minimal levels of transcription.
base
See nitrogenous base.
base analog
Chemical substance that has a structure similar to that of one of the four standard bases of DNA and may be incorporated into newly synthesized DNA molecules in replication.
base-excision repair
DNA repair that first excises modified bases and then replaces the entire nucleotide.
base substitution
Mutation in which a single pair of bases in DNA is altered.
B cell
Particular type of lymphocyte that produces humoral immunity; matures in the bone marrow and produces antibodies.
B-DNA
Right-handed helical structure of DNA that exists when water is abundant; the secondary structure described by Watson and Crick and probably the most common DNA structure in cells.
behavioral isolation
Reproductive isolation due to differences in behavior that prevent interbreeding.
bidirectional replication
Replication at both ends of a replication bubble.
bioinformatics
Synthesis of molecular biology and computer science that develops databases and computational tools to store, retrieve, and analyze nucleic-acid- and protein-sequence data.
biological species concept
Defines a species as a group of organisms whose members are capable of interbreeding with one another but are reproductively isolated from the members of other species. Because different species do not exchange genes, each species evolves independently. Not all biologists adhere to this concept.
biotechnology
Use of biological processes, particularly molecular genetics and recombinant DNA technology, to produce products of commercial value.
bithorax complex
Cluster of three homeotic genes in fruit flies that influences the adult fly’s posterior thoracic and abdominal segments.
bivalent
Refers to a synapsed pair of homologous chromosomes.
blending inheritance
Early concept of heredity proposing that offspring possess a mixture of the traits from both parents.
branch
Evolutionary connections between organisms in a phylogenetic tree.
branch migration
Movement of a cross bridge along two DNA molecules.
branch point
Adenine nucleotide in nuclear pre-mRNA introns that lies from 18 to 40 nucleotides upstream of the 3′ splice site.
broad-sense heritability
Proportion of the phenotypic variance that can be attributed to genetic variance.
cap-binding complex
A group of proteins in eukaryotes that binds to the 5′ cap and initiates translation. Aids in exporting mRNA from the nucleus to the cytoplasm and promotes the initial (pioneer) round of translation.
caspase
Enzyme that cleaves other proteins and regulates apoptosis. Each caspase is synthesized as a large, inactive precursor (a procaspase) that is activated by cleavage, often by another caspase.
catabolite activator protein (CAP)
Protein that functions in catabolite repression. When bound with cAMP, CAP binds to the promoter of certain operons and stimulates transcription.
catabolite repression
System of gene control in some bacterial operons in which glucose is used preferentially and the metabolism of other sugars is repressed in the presence of glucose.
cDNA (complementary DNA) library
Collection of bacterial colonies or phage colonies containing DNA fragments that have been produced by reverse transcription of cellular mRNA.
cell cycle
Stages through which a cell passes from one cell division to the next.
cell line
Genetically identical cells that divide indefinitely and can be cultured in the laboratory.
cell theory
States that all life is composed of cells, that cells arise only from other cells, and that the cell is the fundamental unit of structure and function in living organisms.
cellular immunity
Type of immunity resulting from T cells, which recognize antigens found on the surfaces of self cells.
centiMorgan
Another name for map unit.
central dogma
Concept that genetic information passes from DNA to RNA to protein in a one-way information pathway.
centriole
Cytoplasmic organelle consisting of microtubules; present at each pole of the spindle apparatus in animal cells.
centromere
Constricted region on a chromosome that stains less strongly than the rest of the chromosome; region where spindle microtubules attach to a chromosome.
centromeric sequence
DNA sequence found in functional centromeres.
centrosome
Structure from which the spindle apparatus develops: contains the centriole.
Chargaff's rules
Rules developed by Erwin Chargaff and his colleagues concerning the ratios of bases in DNA.
checkpoint
A key transition point at which progression to the next stage in the cell cycle is regulated.
chiasma
(pl., chiasmata) Point of attachment between homologous chromosomes at which crossing over took place.
chi-square goodness of fit test
Statistical test used to evaluate how well a set of observed values fit the expected values. The probability associated with a calculated chi-square value is the probability that the differences between the observed and the expected values may be due to chance.
chloroplast DNA (cpDNA)
DNA in chloroplasts; has many characteristics in common with eubacterial DNA and typically consists of a circular molecule that lacks histone proteins and encodes some of the rRNAs, tRNAs, and proteins found in chloroplasts.
chorionic villus sampling (CVS)
Procedure used for prenatal genetic testing in which a small piece of the chorion (the outer layer of the placenta) is removed from a pregnant woman. A catheter is inserted through the vagina and cervix into the uterus. Suction is then applied to remove the sample.
chromatin
Material found in the eukaryotic nucleus; consists of DNA and proteins.
chromatin-remodeling complex
Complex of proteins that alters chromatin structure without acetylating histone proteins.
chromatin-remodeling protein
Binds to a DNA sequence and disrupts chromatin structure, causing the DNA to become more accessible to RNA polymerase and other proteins.
chromosomal puff
Localized swelling of a polytene chromosome; a region of chromatin in which DNA has unwound and is undergoing transcription.
chromosome
Structure consisting of DNA and associated proteins that carries and transmits genetic information.
chromosome deletion
Loss of a chromosome segment.
chromosome duplication
Mutation that doubles a segment of a chromosome.
chromosome inversion
Rearrangement in which a segment of a chromosome has been inverted 180 degrees.
chromosome jumping
Method of moving from a gene on a cloned fragment to sequences on distantly linked fragments.
chromosome mutation
Difference from the wild type in the number or structure of one or more chromosomes; often affects many genes and has large phenotypic effects.
chromosome rearrangement
Change from the wild type in the structure of one or more chromosomes.
chromosome theory of heredity
States that genes are located on chromosomes.
chromosome walking
Method of locating a gene by using partly overlapping genomic clones to move in steps from a previously cloned, linked gene to the gene of interest.
cis configuration
Arrangement in which two or more wild-type genes are on one chromosome and their mutant alleles are on the homologous chromosome; also called coupling configuration.
cladogenesis
Evolution in which one lineage is split into two.
clonal evolution
Process by which mutations that enhance the ability of cells to proliferate predominate in a clone of cells, allowing the clone to become increasingly rapid in growth and increasingly aggressive in proliferation properties.
cloning vector
Stable, replicating DNA molecule to which a foreign DNA fragment can be attached and transferred to a host cell.
cloverleaf structure
Secondary structure common to all tRNAs.
coactivator
Protein that cooperates with an activator of transcription. In eukaryotic transcriptional control, coactivators often physically interact with transcriptional activators and the basal transcription apparatus.
codominance
Type of allelic interaction in which the heterozygote simultaneously expresses traits of both homozygotes.
codon
Sequence of three nucleotides that encodes one amino acid in a protein.
coefficient of coincidence
Ratio of observed double crossovers to expected double crossovers.
cohesin
Molecule that holds the two sister chromatids of a chromosome together. The breakdown of cohesin at the centromeres enables the chromatids to separate in anaphase of mitosis and anaphase II of meiosis.
cohesive end
Short, single-stranded overhanging end on a DNA molecule produced when the DNA is cut by certain restriction enzymes. Cohesive ends are complementary and can spontaneously pair to rejoin DNA fragments that have been cut with the same restriction enzyme.
colinearity
Concept that there is a direct correspondence between the nucleotide sequence of a gene and the continuous sequence of amino acids in a protein.
colony
See bacterial colony.
comparative genomics
Comparative studies of the genomes of different organisms.
competent cell
Capable of taking up DNA from its environment (capable of being transformed).
complementary DNA strands
The relation between the two nucleotide strands of DNA in which each purine on one strand pairs with a specific pyrimidine on the opposite strand (A pairs with T, and G pairs with C).
complementation
Two different mutations in the heterozygous condition are exhibited as the wild-type phenotype; indicates that the mutations are at different loci.
complementation test
Test designed to determine whether two different mutations are at the same locus (are allelic) or at different loci (are nonallelic). Two individuals that are homozygous for two independently derived mutations are crossed, producing F1 progeny that are heterozygous for the mutations. If the mutations are at the same locus, the F1 will have a mutant phenotype. If the mutations are at different loci, the F1 will have a wild-type phenotype.
complete dominance
Refers to an allele or a phenotype that is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote phenotype.
complete linkage
Linkage between genes that are located close together on the same chromosome with no crossing over between them.
complete medium
Used to culture bacteria or some other microorganism; contains all the nutrients required for growth and synthesis, including those normally synthesized by the organism. Nutritional mutants can grow on complete medium.
composite transposon
Type of transposable element in bacteria that consists of two insertion sequences flanking a segment of DNA.
compound heterozygote
An individual with two different recessive alleles at a locus that results in a recessive phenotype.
concept of dominance
Principle of heredity discovered by Mendel stating that, when two different alleles are present in a genotype, only one allele may be expressed in the phenotype. The dominant allele is the allele that is expressed, and the recessive allele is the allele that is not expressed.
concordance
Percentage of twin pairs in which both twins have a particular trait.
concordant
Refers to a pair of twins both of whom have the trait under consideration.
conditional mutation
Expressed only under certain conditions.
conditional probability
Probability that is modified by additional information that another event has occurred.
conjugation
Mechanism by which genetic material can be exchanged between bacterial cells. In conjugation, two bacteria lie close together and a cytoplasmic connection forms between them. A plasmid or sometimes a part of the bacterial chromosome passes through this connection from one cell to the other.
consanguinity
Mating between related individuals.
consensus sequence
Comprises the most commonly encountered nucleotides found at a specific location in DNA or RNA.
−10 consensus sequence (Pribnow box)
Consensus sequence (TATAAT) found in most bacterial promoters approximately 10 bp upstream of the transcription start site.
−35 consensus sequence
Consensus sequence (TTGACA) found in many bacterial promoters approximately 35 bp upstream of the transcription start site.
constitutive gene
A gene that is not regulated and is expressed continually.
constitutive mutation
Causes the continuous transcription of one or more structural genes.
contig
Set of overlapping DNA fragments that have been assembled in the correct order to form a continuous stretch of DNA sequence.
continuous characteristic
Displays a large number of possible phenotypes that are not easily distinguished, such as human height.
continuous replication
Replication of the leading strand in the same direction as that of unwinding, allowing new nucleotides to be added continuously to the 3′ end of the new strand as the template is exposed.
coordinate induction
Simultaneous synthesis of several enzymes that is stimulated by a single environmental factor.
copy-number variation (CNV)
Difference among individual organisms in the number of copies of any large DNA sequence (larger than 1000 bp).
core enzyme
Part of bacterial RNA polymerase that, during transcription, catalyzes the elongation of the RNA molecule by the addition of RNA nucleotides; consists of four subunits: two copies of alpha (α), a single copy of beta (β), and a single copy of beta prime (β′).
corepressor
Substance that inhibits transcription in a repressible system of gene regulation; usually a small molecule that binds to a repressor protein and alters it so that the repressor is able to bind to DNA and inhibit transcription.
core promoter
Located immediately upstream of the eukaryotic promoter, DNA sequences to which the basal transcription apparatus binds.
correlation
Degree of association between two or more variables.
correlation coefficient
Statistic that measures the degree of association between two or more variables. A correlation coefficient can range from −1 to +1. A positive value indicates a direct relation between the variables; a negative correlation indicates an inverse relation. The absolute value of the correlation coefficient provides information about the strength of association between the variables.
cosmid
Cloning vector that combines the properties of plasmids and phage vectors and is used to clone large pieces of DNA in bacteria. A cosmid is a small plasmid that carries a λ cos site, allowing the plasmid to be packaged into a viral coat.
cotransduction
Process in which two or more genes are transferred together from one bacterial cell to another. Only genes located close together on a bacterial chromosome will be cotransduced.
cotransformation
Process in which two or more genes are transferred together during cell transformation.
coupling configuration
See cis configuration.
CpG island
DNA region that contains many copies of a cytosine base followed by a guanine base; often found near transcription start sites in eukaryotic DNA. The cytosine bases in CpG islands are commonly methylated when genes are inactive but are demethylated before the initiation of transcription.
CRISPR RNAs (crRNAs)
Small RNA molecules found in prokaryotes that assist in the destruction of foreign DNA.
cross bridge
In a heteroduplex DNA molecule, the point at which each nucleotide strand passes from one DNA molecule to the other.
crossing over
Exchange of genetic material between homologous but nonsister chromatids.
cruciform
Structure formed by the pairing of inverted repeats on both strands of double-stranded DNA.
C value
Haploid amount of DNA found in a cell of an organism.
C-value paradox
The absence of a relationship between genome size and complexity in eukaryotic organisms and the question of what is the function of extra DNA in species with large genome sizes.
cyclin
A key protein in the control of the cell cycle; combines with a cyclin-dependent kinase (CDK). The levels of cyclin rise and fall in the course of the cell cycle.
cyclin-dependent kinase (CDK)
A key protein in the control of the cell cycle; combines with cyclin.
cytokinesis
Process by which the cytoplasm of a cell divides.
cytoplasmic inheritance
Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by only one parent, most cytoplasmically inherited characteristics are inherited from a single parent.
cytosine (C)
Pyrimidine base in DNA and RNA.
deamination
Loss of an amino group (NH2) from a base.
degenerate genetic code
Refers to the fact that the genetic code contains more information than is needed to specify all 20 common amino acids.
deletion
Mutation in which one or more nucleotides are deleted from a DNA sequence.
deletion mapping
Technique for determining the chromosomal location of a gene by studying the association of its phenotype or product with particular chromosome deletions.
denaturation (melting)
Process that separates the strands of double-stranded DNA when DNA is heated.
deoxyribonucleotide
Basic building block of DNA, consisting of deoxyribose, a phosphate, and a nitrogenous base.
deoxyribose
Five-carbon sugar in DNA; lacks a hydroxyl group on the 2′-carbon atom.
depurination
Break in the covalent bond connecting a purine base to the 1′-carbon atom of deoxyribose, resulting in the loss of the purine base.
determination
Process by which a cell becomes committed to developing into a particular cell type.
diakinesis
Fifth substage of prophase I in meiosis. In diakinesis, chromosomes contract, the nuclear membrane breaks down, and the spindle forms.
dicentric bridge
Structure produced when the two centromeres of a dicentric chromatid are pulled toward opposite poles, stretching the dicentric chromosome across the center of the nucleus. Eventually, the dicentric bridge breaks as the two centromeres are pulled apart.
dicentric chromatid
Chromatid that has two centromeres; produced when crossing over takes place within a paracentric inversion. The two centromeres of the dicentric chromatid are frequently pulled toward opposite poles in mitosis or meiosis, breaking the chromosome.
dideoxyribonucleoside triphosphate (ddNTP)
Special substrate for DNA synthesis used in the Sanger dideoxy sequencing method; identical with dNTP (the usual substrate for DNA synthesis) except that it lacks a 3′-OH group. The incorporation of a ddNTP into DNA terminates DNA synthesis.
dihybrid cross
A cross between two individuals that differ in two characteristics—more specifically, a cross between individuals that are homozygous for different alleles at the two loci (AA BB × aa bb); also refers to a cross between two individuals that are both heterozygous at two loci (Aa Bb × Aa Bb).
dioecious organism
Belongs to a species whose members have either male or female reproductive structures.
diploid
Possessing two sets of chromosomes (two genomes).
diplotene
Fourth substage of prophase I in meiosis. In diplotene, centromeres of homologous chromosomes move apart, but the homologs remain attached at chiasmata.
directional selection
Selection in which one trait or allele is favored over another.
direct repair
DNA repair in which modified bases are changed back into their original structures.
direct-to-consumer genetic test
Test for a genetic condition; the test can be purchased directly by a consumer, without the involvement of a physician or other health care provider.
discontinuous characteristic
Exhibits only a few, easily distinguished phenotypes. An example is seed shape in which seeds are either round or wrinkled.
discontinuous replication
Replication of the lagging strand in the direction opposite that of unwinding, which means that DNA must be synthesized in short stretches (Okazaki fragments).
discordant
Refers to a pair of twins of whom one twin has the trait under consideration and the other does not.
displaced duplication
Chromosome rearrangement in which the duplicated segment is some distance from the original segment, either on the same chromosome or on a different one.
dizygotic twins
Nonidentical twins that arise when two different eggs are fertilized by two different sperm; also called fraternal twins.
D loop
Region of mitochondrial DNA that contains an origin of replication and promoters; it is displaced during the initiation of replication, leading to the name displacement, or D, loop.
DNA fingerprinting
Technique used to identify individuals by examining their DNA sequences.
DNA gyrase
E. coli topoisomerase enzyme that relieves the torsional strain that builds up ahead of the replication fork.
DNA helicase
Enzyme that unwinds double-stranded DNA by breaking hydrogen bonds.
DNA library
Collection of bacterial colonies containing all the DNA fragments from one source.
DNA ligase
Enzyme that catalyzes the formation of a phosphodiester bond between adjacent 3′-OH and 5′-phosphate groups in a DNA molecule.
DNA methylation
Modification of DNA by the addition of methyl groups to certain positions on the bases.
DNA polymerase
Enzyme that synthesizes DNA.
DNA polymerase I
Bacterial DNA polymerase that removes and replaces RNA primers with DNA nucleotides.
DNA polymerase II
Bacterial DNA polymerase that takes part in DNA repair; restarts replication after synthesis has halted because of DNA damage.
DNA polymerase III
Bacterial DNA polymerase that synthesizes new nucleotide strands by using the 3′-OH group provided by the primer.
DNA polymerase IV
Bacterial DNA polymerase; probably takes part in DNA repair.
DNA polymerase V
Bacterial DNA polymerase; probably takes part in DNA repair.
DNA polymerase α
Eukaryotic DNA polymerase that initiates replication.
DNA polymerase β
Eukaryotic DNA polymerase that participates in DNA repair.
DNA polymerase δ
Eukaryotic DNA polymerase that replicates the lagging strand during DNA synthesis; also carries out DNA repair and translesion DNA synthesis.
DNA polymerase ε
Eukaryotic DNA polymerase that replicates the leading strand during DNA synthesis.
DNA polymerase γ
Eukaryotic DNA polymerase that replicates mitochondrial DNA. A γ-like DNA polymerase replicates chloroplast DNA.
DNase I hypersensitive site
Chromatin region that becomes sensitive to digestion by the enzyme DNase I.
DNA sequencing
Process of determining the sequence of bases along a DNA molecule.
DNA transposon
See transposable element.
domain
Functional part of a protein.
dominance genetic variance
Component of the genetic variance that can be attributed to dominance (interaction between genes at the same locus).
dominant
Refers to an allele or a phenotype that is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote phenotype.
dosage compensation
Equalization in males and females of the amount of protein produced by X-linked genes. In placental mammals, dosage compensation is accomplished by the random inactivation of one X chromosome in the cells of females.
double fertilization
Fertilization in plants; includes the fusion of a sperm cell with an egg cell to form a zygote and the fusion of a second sperm cell with the polar nuclei to form an endosperm.
double-strand-break model
Model of homologous recombination in which a DNA molecule undergoes double-strand breaks.
down mutation
Decreases the rate of transcription.
downstream core promoter element
Consensus sequence [RG(A or T)CGTG] found in some eukaryotic RNA polymerase II core promoters; usually located approximately 30 bp downstream of the transcription start site.
Down syndrome (trisomy 21)
Characterized by variable degrees of intellectual disability, characteristic facial features, some retardation of growth and development, and an increased incidence of heart defects, leukemia, and other abnormalities; caused by the duplication of all or part of chromosome 21.
ecological isolation
Reproductive isolation in which different species live in different habitats and interact with the environment in different ways. Thus, their members do not encounter one another and do not reproduce with one another.
Edward syndrome (trisomy 18)
Characterized by severe intellectual disability, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities; results from the presence of three copies of chromosome 18.
effective population size
Effective number of breeding adults in a population; influenced by the number of individuals contributing genes to the next generation, their sex ratio, variation between individuals in reproductive success, fluctuations in population size, the age structure of the population, and whether mating is random.
egg-polarity gene
Determines the major axes of development in an early fruit-fly embryo. One set of egg-polarity genes determines the anterior–posterior axis and another determines the dorsal–ventral axis.
elongation factor G (EF-G)
Protein that combines with GTP and is required for movement of the ribosome along the mRNA during translation.
elongation factor Ts (EF-Ts)
Protein that regenerates elongation factor Tu in the elongation stage of protein synthesis.
elongation factor Tu (EF-Tu)
Protein taking part in the elongation stage of protein synthesis; forms a complex with GTP and a charged amino acid and then delivers the charged tRNA to the ribosome.
endosymbiotic theory
States that some membrane-bounded organelles, such as mitochondria and chloroplasts, in eukaryotic cells originated as free-living eubacterial cells that entered into an endosymbiotic relation with a eukaryotic host cell and evolved into the present-day organelles; supported by a number of similarities in structure and sequence between organelle and eubacterial DNAs.
enhancer
Sequence that stimulates maximal transcription of distant genes; affects only genes on the same DNA molecule (is cis acting), contains short consensus sequences, is not fixed in relation to the transcription start site, can stimulate almost any promoter in its vicinity, and may be upstream or downstream of the gene. The function of an enhancer is independent of sequence orientation.
environmental variance
Component of the phenotypic variance that is due to environmental differences among individual members of a population.
epialleles
Alleles that do not differ in their base sequence but have epigenetic differences that produce heritable variations in phenotypes.
epigenetic change
Stable alteration of chromatin structure that may be passed on to other cells or to an individual organism. See also epigenetics.
epigenetic marks
Heritable changes to DNA or chromatin structure that do not involve alteration of the base sequence and that bring about changes in gene expression.
epigenetics
Phenomena due to alterations to DNA that do not include changes in the base sequence; often affect the way in which the DNA sequences are expressed. Such alterations are often stable and heritable in the sense that they are passed from one cell to another.
epigenome
All epigenetic modifications within the genome of an individual organism.
engineered nucelease
Protein consisting of part of a restriction enzyme that cleaves DNA combined with another protein that recognizes and binds to a specific DNA sequence. Capable of making double-stranded cuts to the DNA at a predetermined DNA sequence. Engineered nucleases can be custom designed to bind to and cut any particular DNA sequence.
episome
Plasmid capable of integrating into a bacterial chromosome.
epistasis
Type of gene interaction in which a gene at one locus masks or suppresses the effects of a gene at a different locus.
epistatic gene
Masks or suppresses the effect of a gene at a different locus.
equilibrium
Situation in which no further change takes place; in population genetics, refers to a population in which allelic frequencies do not change.
equilibrium density gradient centrifugation
Method used to separate molecules or organelles of different density by centrifugation.
eubacteria
One of the three primary divisions of life. Eubacteria consist of unicellular organisms with prokaryotic cells and include most of the common bacteria.
euchromatin
Chromatin that undergoes condensation and decondensation in the course of the cell cycle.
eukaryote
Organism with a complex cell structure including a nuclear envelope and membrane-bounded organelles. One of the three primary divisions of life, eukaryotes include unicellular and multicellular forms.
evolution
Genetic change taking place in a group of organisms.
exit (E) site
One of three sites in a ribosome occupied by a tRNA. In the elongation stage of translation, the tRNA moves from the peptidyl (P) site to the E site from which it then exits the ribosome.
exon
Coding region of a split gene (a gene that is interrupted by introns). After processing, the exons remain in messenger RNA.
exon shuffling
Process, important in the evolution of eukaryotic genes, by which exons of different genes are exchanged and mixed into new combinations, creating new genes that are mosaics of other preexisting genes.
expanding nucleotide repeat
Mutation in which the number of copies of a set of nucleotides (most often three nucleotides) increases in succeeding generations.
expressed-sequence tag (EST)
Unique fragment of DNA from the coding region of a gene, produced by the reverse transcription of cellular RNA. Parts of the fragments are sequenced so that they can be identified.
expression vector
Cloning vector containing DNA sequences such as a promoter, a ribosome-binding site, and transcription initiation and termination sites that allow DNA fragments inserted into the vector to be transcribed and translated.
expressivity
Degree to which a trait is expressed.
familial Down syndrome
Caused by a Robertsonian translocation in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families.
fertilization
Fusion of gametes (sex cells) to form a zygote.
fetal cell sorting
Separation of fetal cells from maternal blood. Genetic testing on the fetal cells can provide information about genetic diseases and disorders in the fetus.
F (fertility) factor
Episome of E. coli that controls conjugation and gene exchange between E. coli cells. The F factor contains an origin of replication and genes that enable the bacterium to undergo conjugation.
F1 (first filial) generation
Offspring of the initial parents (P) in a genetic cross.
F2 (second filial) generation
Offspring of the F1 generation in a genetic cross; the third generation of a genetic cross.
first polar body
One of the products of meiosis I in oogenesis; contains half the chromosomes but little of the cytoplasm.
fitness
Reproductive success of a genotype compared with that of other genotypes in a population.
5-methylcytosine
Modified form of the base cytosine, containing a methyl group (CH3) on the 5′ carbon.
5′ cap
Modified 5′ end of eukaryotic mRNA, consisting of an extra nucleotide (methylated) and methylation of the 2′ position of the ribose sugar in one or more subsequent nucleotides; plays a role in the binding of the ribosome to mRNA and affects mRNA stability and the removal of introns.
5′ end
End of the polynucleotide chain where a phosphate is attached to the 5′-carbon atom of the nucleotide.
5′ splice site
The 5′ end of an intron where cleavage takes place in RNA splicing.
5′ untranslated (5′ UTR) region
Sequence of nucleotides at the 5′ end of mRNA; does not encode the amino acids of a protein.
fixation
Point at which one allele reaches a frequency of 1. At this point, all members of the population are homozygous for the same allele.
flanking direct repeat
Short, directly repeated sequence produced on either side of a transposable element when the element inserts into DNA.
forward genetics
Traditional approach to the study of gene function that begins with a phenotype (a mutant organism) and proceeds to a gene that encodes the phenotype.
forward mutation
Alters a wild-type phenotype.
founder effect
Sampling error that arises when a population is established by a small number of individuals; leads to genetic drift.
fragile site
Constriction or gap that appears at a particular location on a chromosome when cells are cultured under special conditions. One fragile site on the human X chromosome is associated with intellectual disability (fragile-X syndrome) and results from an expanding trinucleotide repeat.
fragile-X syndrome
A form of X-linked intellectual disability that appears primarily in males; results from an increase in the number of repeats of a CGG trinucleotide.
frameshift mutation
Alters the reading frame of a gene.
fraternal twins
Nonidentical twins that arise when two different eggs are fertilized by two different sperm; also called dizygotic twins.
frequency distribution
Graphical way of representing values. In genetics, usually the phenotypes found in a group of individuals are displayed as a frequency distribution. Typically, the phenotypes are plotted on the horizontal (x) axis and the numbers (or proportions) of individuals with each phenotype are plotted on the vertical (y) axis.
functional genomics
Area of genomics that studies the functions of genetic information contained within genomes.
G0 (gap 0)
Nondividing stage of the cell cycle.
G1 (gap 1)
Stage in interphase of the cell cycle in which the cell grows and develops.
G2 (gap 2)
Stage of interphase in the cell cycle that follows DNA replication. In G2, the cell prepares for division.
gain-of-function mutation
Produces a new trait or causes a trait to appear in inappropriate tissues or at inappropriate times in development.
gametic isolation
Reproductive isolation due to the incompatibility of gametes. Mating between members of different species takes place, but the gametes do not form zygotes. Seen in many plants, where pollen from one species cannot fertilize the ovules of another species.
gametophyte
Haploid phase of the life cycle in plants.
gap genes
In fruit flies, set of segmentation genes that define large sections of the embryo. Mutations in these genes usually eliminate whole groups of adjacent segments.
gel electrophoresis
Technique for separating charged molecules (such as proteins or nucleic acids) on the basis of molecular size or charge or both.
gene
Genetic factor that helps determine a trait; often defined at the molecular level as a DNA sequence that is transcribed into an RNA molecule.
gene cloning
Insertion of DNA fragments into bacteria in such a way that the fragments will be stable and copied by the bacteria.
gene conversion
Process of nonreciprocal genetic exchange that can produce abnormal ratios of gametes following meiosis.
gene desert
In reference to the density of genes in the genome, a region that is gene poor—that is, a long stretch of DNA possibly consisting of hundreds of thousands to millions of base pairs completely devoid of any known genes or other functional sequences.
gene family
See multigene family.
gene flow
Movement of genes from one population to another; also called migration.
gene interaction
Interaction between genes at different loci that affect the same characteristic.
gene mutation
Affects a single gene or locus.
gene pool
Total of all genes in a population.
generalized transduction
Transduction in which any gene can be transferred from one bacterial cell to another by a virus.
general transcription factor
Protein that binds to eukaryotic promoters near the start site and is a part of the basal transcription apparatus that initiates transcription.
gene regulation
Mechanisms and processes that control the phenotypic expression of genes.
gene therapy
Use of recombinant DNA to treat a disease or disorder by altering the genetic makeup of the patient’s cells.
genetic bottleneck
Sampling error that arises when a population undergoes a drastic reduction in population size; leads to genetic drift.
genetic-conflict hypothesis
Suggests that genomic imprinting evolved because different and conflicting pressures act on maternal and paternal alleles for genes that affect fetal growth. For example, paternally derived alleles often favor maximum fetal growth, whereas maternally derived alleles favor less than maximum fetal growth because of the high cost of fetal growth to the mother.
genetic correlation
Phenotypic correlation due to the same genes affecting two or more characteristics.
genetic counseling
Educational process that attempts to help patients and family members deal with all aspects of a genetic condition.
genetic drift
Change in allelic frequency due to sampling error.
genetic engineering
Common term for recombinant DNA technology.
genetic–environmental interaction variance
Component of the phenotypic variance that results from an interaction between genotype and environment. Genotypes are expressed differently in different environments.
Genetic Information Nondiscrimination Act (GINA)
U.S. law prohibiting health insurers from using genetic information to make decisions about health-insurance coverage and rates; prevents employers from using genetic information in employment decisions; also prevents health insurers and employers from asking or requiring a person to take a genetic test.
genetic (linkage) map
Map of the relative distances between genetic loci, markers, or other chromosome regions determined by rates of recombination; measured in percent recombination or map units.
genetic marker
Any gene or DNA sequence used to identify a location on a genetic or physical map.
genetic maternal effect
Determines the phenotype of an offspring. With genetic maternal effect, an offspring inherits genes for the characteristics from both parents, but the offspring’s phenotype is determined not by its own genotype but by the nuclear genotype of its mother.
genetic rescue
Introduction of new genetic variation into an inbred population that often dramatically improves the health of the population in an effort to increase its chances of long-term survival.
genetic variance
Component of the phenotypic variance that is due to genetic differences among individual members of a population.
gene tree
Phylogenetic tree representing the evolutionary relationships among a set of genes.
genic interaction variance
Component of the genetic variance that can be attributed to genic interaction (interaction between genes at different loci).
genic sex determination
Sex determination in which the sexual phenotype is specified by genes at one or more loci, but there are no obvious differences in the chromosomes of males and females.
genome
Complete set of genetic instructions for an organism.
genome-wide association studies
Studies that look for nonrandom associations between the presence of a trait and alleles at many different loci scattered across a genome—that is, for associations between traits and particular suites of alleles in a population.
genomic imprinting
Differential expression of a gene that depends on the sex of the parent that transmitted the gene. If the gene is inherited from the father, its expression differs from that if the gene is inherited from the mother.
genomic library
Collection of bacterial or phage colonies containing DNA fragments that consist of the entire genome of an organism.
genomics
Study of the content, organization, and function of genetic information in whole genomes.
genotype
The set of genes possessed by an individual organism.
genotypic frequency
Proportion of a particular genotype within a population.
germ-line mutation
Mutation in a germ-line cell (one that gives rise to gametes).
germ-plasm theory
States that cells in the reproductive organs carry a complete set of genetic information.
G2/M checkpoint
Important point in the cell cycle near the end of G2. After this checkpoint has been passed, the cell undergoes mitosis.
G overhang
A guanine-rich sequence of nucleotides that protrudes beyond the complementary C-rich strand at the end of a chromosome.
group I intron
Belongs to a class of introns in some ribosomal RNA genes that are capable of self-splicing.
group II intron
Belongs to a class of introns in some protein-encoding genes that are capable of self-splicing and are found in mitochondria, chloroplasts, and a few eubacteria.
G1/S checkpoint
Important point in the cell cycle. After the G1/S checkpoint has been passed, DNA replicates and the cell is committed to dividing.
guanine (G)
Purine base in DNA and RNA.
guide RNA (gRNA)
RNA molecule that serves as a template for an alteration made in mRNA during RNA editing.
gynandromorph
Individual organism that is a mosaic for the sex chromosomes, possessing tissues with different sex-chromosome constitutions.
hairpin
Secondary structure formed when sequences of nucleotides on the same strand are complementary and pair with each other.
haploid
Possessing a single set of chromosomes (one genome).
haploinsufficiency
The appearance of a mutant phenotype in an individual cell or organism that is heterozygous for a normally recessive trait.
haploinsufficient gene
Must be present in two copies for normal function. If one copy of the gene is missing, a mutant phenotype is produced.
haplotype
A specific set of linked genetic variants or alleles on a single chromosome or on part of a chromosome.
Hardy–Weinberg equilibrium
Frequencies of genotypes when the conditions of the Hardy–Weinberg law are met.
Hardy–Weinberg law
Important principle of population genetics stating that, in a large, randomly mating population not affected by mutation, migration, or natural selection, allelic frequencies will not change and genotypic frequencies stabilize after one generation in the proportions p2 (the frequency of AA), 2pq (the frequency of Aa), and q2 (the frequency of aa), where p equals the frequency of allele A and q equals the frequency of allele a.
H-DNA
DNA structure consisting of three nucleotide strands (triplex DNA). Can occur when a single nucleotide strand from one part of the molecule pairs with double-stranded DNA from another part of the molecule.
heat-shock protein
Produced by many cells in response to extreme heat and other stresses; helps cells prevent damage from such stressing agents.
helicase
See DNA helicase.
hemizygosity
Possession of a single allele at a locus. Males of organisms with XX-XY sex determination are hemizygous for X-linked loci because their cells possess a single X chromosome.
heritability
Proportion of phenotypic variation due to genetic differences. See also broad-sense heritability and narrow-sense heritability.
hermaphroditism
Condition in which an individual organism possesses both male and female reproductive structures. True hermaphrodites produce both male and female gametes.
heterochromatin
Chromatin that remains in a highly condensed state throughout the cell cycle; found at the centromeres and telomeres of most chromosomes.
heteroduplex DNA
DNA consisting of two strands, each of which is from a different chromosome.
heterogametic sex
The sex (male or female) that produces two types of gametes with respect to sex chromosomes. For example, in the XX-XY sex-determining system, the male produces both X-bearing and Y-bearing gametes.
heterokaryon
Cell possessing two nuclei derived from different cells through cell fusion.
heteroplasmy
Presence of two or more distinct variants of DNA within the cytoplasm of a single cell.
heterozygote screening
Tests members of a population to identify heterozygous carriers of a disease-causing allele who are healthy but have the potential to produce children who have the disease.
heterozygous
Refers to an individual organism that possesses two different alleles at a locus.
highly repetitive DNA
DNA that consists of short sequences that are present in hundreds of thousands to millions of copies; clustered in certain regions of chromosomes.
histone
Low-molecular-weight protein found in eukaryotes that complexes with DNA to form chromosomes.
histone code
Modification of histone proteins, such as the addition or removal of phosphate groups, methyl groups, or acetyl groups, that encode information affecting how genes are expressed.
Holliday intermediate
Structure that forms in homologous recombination; consists of two duplex molecules connected by a cross bridge.
Holliday junction
Model of homologous recombination that is initiated by single-strand breaks in a DNA molecule.
holoenzyme
Complex of an enzyme and other protein factors necessary for complete function.
homeobox
Conserved subset of nucleotides in homeotic genes. In Drosophila, it consists of 180 nucleotides that encode 60 amino acids of a DNA-binding domain related to the helix-turn-helix motif.
homeotic complex (HOM-C)
Major cluster of homeotic genes in fruit flies; consists of the Antennapedia complex, which affects the development of the adult fly’s head and anterior segments, and the bithorax complex, which affects the adult fly’s posterior thoracic and abdominal segments.
homeotic gene
Determines the identity of individual segments or parts in an early embryo. Mutations in such genes cause body parts to appear in the wrong places.
homogametic sex
The sex (male or female) that produces gametes that are all alike with regard to sex chromosomes. For example, in the XX-XY sex-determining system, the female produces only X-bearing gametes.
homologous genes
Evolutionarily related genes, having descended from a gene in a common ancestor.
homologous pair of chromosomes
Two chromosomes that are alike in structure and size and that carry genetic information for the same set of hereditary characteristics. One chromosome of a homologous pair is inherited from the male parent and the other is inherited from the female parent.
homologous recombination
Exchange of genetic information between homologous DNA molecules.
homoplasmy
Presence of only one version of DNA within the cytoplasm of a single cell.
homozygous
Refers to an individual organism that possesses two identical alleles at a locus.
horizontal gene transfer
Transfer of genes from one organism to another by a mechanism other than reproduction.
Hox gene
Gene that contains a homeobox.
human papilloma virus (HPV)
Virus associated with cervical cancer.
Human Proteome Project
Project with the goal of identifying and characterizing all proteins in the human body.
humoral immunity
Type of immunity resulting from antibodies produced by B cells.
hybrid breakdown
Reproductive isolating mechanism in which closely related species are capable of mating and producing viable and fertile F1 progeny, but genes do not flow between the two species, because further crossing of the hybrids produces inviable or sterile offspring.
hybrid dysgenesis
Sudden appearance of numerous mutations, chromosome aberrations, and sterility in the offspring of a cross between a male fly that possesses P elements and a female fly that lacks them.
hybrid inviability
Reproductive isolating mechanism in which mating between two organisms of different species takes place and hybrid offspring are produced but are not viable.
hybridization
Pairing of two partly or fully complementary single-stranded nucleotide chains.
hybrid sterility
Hybrid embryos complete development but are sterile; exemplified by mating between donkeys and horses to produce a mule, a viable but usually sterile offspring.
hypostatic gene
Gene that is masked or suppressed by the action of a gene at a different locus.
identical twins
Twins that arise when a single egg fertilized by a single sperm splits into two separate embryos; also called monozygotic twins.
inbreeding
Mating between related individuals that takes place more frequently than expected on the basis of chance.
inbreeding coefficient
Measure of inbreeding; the probability (ranging from 0 to 1) that two alleles are identical by descent.
inbreeding depression
Decreased fitness arising from inbreeding; often due to the increased expression of lethal or deleterious recessive traits.
incomplete dominance
Refers to the phenotype of a heterozygote that is intermediate between the phenotypes of the two homozygotes.
incomplete linkage
Linkage between genes that exhibit some crossing over; intermediate in its effects between independent assortment and complete linkage.
incomplete penetrance
Refers to a genotype that does not always express the expected phenotype. Some individuals possess the genotype for a trait but do not express the phenotype.
incorporated error
Incorporation of a damaged nucleotide or mismatched base pair into a DNA molecule.
independent assortment
Independent separation of chromosome pairs in anaphase I of meiosis; contributes to genetic variation.
induced mutation
Mutation that results from environmental agents, such as chemicals or radiation.
induced pluripotent stem cells (iPSCs)
Adult cells that have been artificially induced to dedifferentiate and revert to pluripotent stem cells capable of becoming many types of cells.
inducer
Substance that stimulates transcription in an inducible system of gene regulation; usually a small molecule that binds to a repressor protein and alters that repressor so that it can no longer bind to DNA and inhibit transcription.
inducible operon
Operon or other system of gene regulation in which transcription is normally off. Something must take place for transcription to be induced, or turned on.
induction
Stimulation of the synthesis of an enzyme by an environmental factor, often the presence of a particular substrate.
in-frame deletion
Deletion of some multiple of three nucleotides, which does not alter the reading frame of the gene.
in-frame insertion
Insertion of some multiple of three nucleotides, which does not alter the reading frame of the gene.
inheritance of acquired characteristics
Early notion of inheritance proposing that acquired traits are passed to descendants.
initiation codon
The codon in mRNA that specifies the first amino acid (fMet in bacterial cells; Met in eukaryotic cells) of a protein; most commonly AUG.
initiation factor 1 (IF-1)
Protein required for the initiation of translation in bacterial cells; enhances the dissociation of the large and small subunits of the ribosome.
initiation factor 2 (IF-2)
Protein required for the initiation of translation in bacterial cells; forms a complex with GTP and the charged initiator tRNA and then delivers the charged tRNA to the initiation complex.
initiation factor 3 (IF-3)
Protein required for the initiation of translation in bacterial cells; binds to the small subunit of the ribosome and prevents the large subunit from binding during initiation.
initiator protein
Binds to an origin of replication and unwinds a short stretch of DNA, allowing helicase and other single-strand-binding proteins to bind and initiate replication.
insertion
Mutation in which nucleotides are added to a DNA sequence.
insertion sequence
Simple type of transposable element found in bacteria and their plasmids that contains only the information necessary for its own movement.
in situ hybridization
Method used to determine the chromosomal location of a gene or other specific DNA fragment or the tissue distribution of an mRNA by using a labeled probe that is complementary to the sequence of interest.
insulator
DNA sequence that blocks or insulates the effect of an enhancer; must be located between the enhancer and the promoter to have blocking activity; also may limit the spread of changes in chromatin structure.
integrase
Enzyme that inserts prophage, or proviral, DNA into a chromosome.
interactome
Complete set of protein interactions in a cell.
intercalating agent
Chemical substance that is about the same size as a nucleotide and may become sandwiched between adjacent bases in DNA, distorting the three-dimensional structure of the helix and causing single-nucleotide insertions and deletions in replication.
interference
Degree to which one crossover interferes with additional crossovers.
intergenic suppressor mutation
Suppressor mutation that occurs in a gene (locus) that is different from the gene containing the original mutation.
interkinesis
Period between meiosis I and meiosis II.
internal promoter
Promoter located within the sequences of DNA that are transcribed into RNA.
interphase
Period in the cell cycle between the cell divisions. In interphase, the cell grows, develops, and prepares for cell division.
interspersed repeat sequence
Repeated sequence at multiple locations throughout the genome.
intrachromosomal recombination
Recombination among genes located on the same chromosome.
intragenic mapping
Maps the locations of mutations within a single locus.
intragenic suppressor mutation
Suppressor mutation that occurs in the same gene (locus) as the mutation that it suppresses.
intron
Intervening sequence in a split gene; removed from the RNA after transcription.
inverted repeats
Sequences on the same strand that are inverted and complementary.
isoaccepting tRNAs
Different tRNAs with different anticodons that specify the same amino acid.
isotopes
Different forms of an element that have the same number of protons and electrons but differ in the number of neutrons in the nucleus.
junctional diversity
Addition or deletion of nucleotides at the junctions of gene segments brought together in the somatic recombination of genes that encode antibodies and T-cell receptors.
karyotype
Picture of an individual organism’s complete set of metaphase chromosomes.
kinetochore
Set of proteins that assemble on the centromere, providing the point of attachment for spindle microtubules.
Klinefelter syndrome
Human condition in which cells contain one or more Y chromosomes along with multiple X chromosomes (most commonly XXY but may also be XXXY, XXXXY, or XXYY). Persons with Klinefelter syndrome are male in appearance but frequently possess small testes, some breast enlargement, and reduced facial and pubic hair; often taller than normal and sterile, most have normal intelligence.
knock-in mouse
Mouse that carries a foreign sequence inserted at a specific chromosome location.
knockout mouse
Mouse in which a normal gene has been disabled (“knocked out”).
labeling
Method for adding a radioactive or chemical label to a molecule such as DNA or RNA.
lagging strand
DNA strand that is replicated discontinuously.
large ribosomal subunit
The larger of the two subunits of a functional ribosome.
lariat
Looplike structure created in the splicing of nuclear pre-mRNA in which the 5′ end of an intron is attached to a branch point in pre-mRNA.
leading strand
DNA strand that is replicated continuously.
leptotene
First substage of prophase I in meiosis. In leptotene, chromosomes contract and become visible.
lethal allele
Causes the death of an individual organism, often early in development, and so the organism does not appear in the progeny of a genetic cross. Recessive lethal alleles kill individual organisms that are homozygous for the allele; dominant lethals kill both heterozygotes and homozygotes.
lethal mutation
Causes premature death.
LINE
See long interspersed element.
linkage analysis
Gene mapping based on the detection of physical linkage between genes, as measured by the rate of recombination, in progeny from a cross.
linkage disequilibrium
Nonrandom association between genetic variants within a haplotype.
linkage group
Genes located together on the same chromosome.
linked genes
Genes located on the same chromosome.
linker
Small, synthetic DNA fragment that contains one or more restriction sites. Can be attached to the ends of any piece of DNA and used to insert a gene into a plasmid vector when restriction sites are not available.
linker DNA
Stretch of DNA separating two nucleosomes.
locus
Position on a chromosome where a specific gene is located.
lod (logarithm of odds) score
Logarithm of the ratio of the probability of obtaining a set of observations, assuming a specified degree of linkage, to the probability of obtaining the same set of observations with independent assortment; used to assess the likelihood of linkage between genes from pedigree data.
long interspersed element (LINE)
Long DNA sequence repeated many times and interspersed throughout the genome.
loss-of-function mutation
Causes the complete or partial absence of normal function.
loss of heterozygosity
At a locus having a normal allele and a mutant allele, inactivation or loss of the normal allele.
Lyon hypothesis
Proposed by Mary Lyon in 1961, this hypothesis proposes that one X chromosome in each female cell becomes inactivated (a Barr body) and suggests that which of the X chromosomes becomes inactivated is random and varies from cell to cell.
lysogenic cycle
Life cycle of a bacteriophage in which phage genes first integrate into the bacterial chromosome and are not immediately transcribed and translated.
lytic cycle
Life cycle of a bacteriophage in which phage genes are transcribed and translated, new phage particles are produced, and the host cell is lysed.
major histocompatibility complex (MHC) antigen
Belongs to a large and diverse group of antigens found on the surfaces of cells that mark those cells as self; encoded by a large cluster of genes known as the major histocompatibility complex. T cells simultaneously bind to foreign and MHC antigens.
malignant tumor
Consists of cells that are capable of invading other tissues.
map-based sequencing
Method of sequencing a genome in which sequenced fragments are ordered into contigs with the use of genetic or physical maps.
mapping function
Relates recombination frequencies to actual physical distances between genes.
map unit (m.u.)
Unit of measure for distances on a genetic map; 1 map unit equals 1% recombination.
mass spectrometry
Method for precisely determining the molecular mass of a molecule by using the migration rate of an ionized molecule in an electrical field.
maternal blood screening test
Tests for genetic conditions in a fetus by analyzing the blood of the mother. For example, the level of α-fetoprotein in maternal blood provides information about the probability that a fetus has a neural-tube defect.
mean
Statistic that describes the center of a distribution of measurements; calculated by dividing the sum of all measurements by the number of measurements; also called the average.
mechanical isolation
Reproductive isolation resulting from anatomical differences that prevent successful copulation.
mediator
Complex of proteins that is one of the components of the basal transcription apparatus.
megaspore
One of the four products of meiosis in plants.
megasporocyte
In the ovary of a plant, a diploid reproductive cell that undergoes meiosis to produce haploid macrospores.
meiosis
Process in which chromosomes of a eukaryotic cell divide to give rise to haploid reproductive cells. Consists of two divisions: meiosis I and meiosis II.
meiosis I
First phase of meiosis. In meiosis I, chromosome number is reduced by half.
meiosis II
Second phase of meiosis. Events in meiosis II are similar to those in mitosis.
melting
See denaturation.
melting temperature
Midpoint of the melting range of DNA.
memory cell
Long-lived lymphocyte among the clone of cells generated when a foreign antigen is encountered. If the same antigen is encountered again, the memory cells quickly divide and give rise to another clone of cells specific for that particular antigen.
Mendelian population
Group of interbreeding, sexually reproducing individuals.
meristic characteristic
Characteristic whose phenotype varies in whole numbers, such as number of vertebrae.
merozygote
Bacterial cell that has two copies of some genes—one copy on the bacterial chromosome and a second copy on an introduced F plasmid; also called partial diploid.
messenger RNA (mRNA)
RNA molecule that carries genetic information for the amino acid sequence of a protein.
metacentric chromosome
Chromosome in which the two chromosome arms are approximately the same length.
metagenomics
An emerging field of sequencing technology in which the genome sequences of a group of organisms inhabiting a common environment are sampled and determined.
metaphase
Stage of mitosis. In metaphase, chromosomes align in the center of the cell.
metaphase I
Stage of meiosis I. In metaphase I, homologous pairs of chromosomes align in the center of the cell.
metaphase II
Stage of meiosis II. In metaphase II, individual chromosomes align on the metaphase plate.
metaphase plate
Plane in a cell between two spindle poles. In metaphase, chromosomes align on the metaphase plate.
metastasis
Refers to cells that separate from malignant tumors and travel to other sites, where they establish secondary tumors.
5′-methylcytosine
Modified nucleotide, consisting of cytosine to which a methyl group has been added; predominate form of methylation in eukaryotic DNA.
microarray
Ordered array of DNA fragments fixed to a solid support, which serve as probes to detect the presence of complementary sequences; often used to assess the expression of genes in various tissues and under different conditions.
microbiome
Complete set of all bacteria found in a particular environment. For example, the human microbiome consists of all bacteria found in and on the human body.
microRNA (miRNA)
Small RNA molecule, typically 21 or 22 bp in length, produced by cleavage of double-stranded RNA arising from small hairpins within RNA that is mostly single stranded. The miRNAs combine with proteins to form a complex that binds (imperfectly) to mRNA molecules and inhibits their translation.
microsatellite
See variable number of tandem repeats.
microspore
Haploid product of meiosis in plants.
microsporocyte
Diploid reproductive cell in the stamen of a plant; undergoes meiosis to produce four haploid microspores.
microtubule
Long fiber composed of the protein tubulin; plays an important role in the movement of chromosomes in mitosis and meiosis.
migration
Movement of genes from one population to another; also called gene flow.
minimal medium
Used to culture bacteria or some other microorganism; contains only the nutrients required by prototrophic (wild-type) cells—typically, a carbon source, essential elements such as nitrogen and phosphorus, certain vitamins, and other required ions and nutrients.
mismatch repair
Process that corrects mismatched nucleotides in DNA after replication has been completed. Enzymes excise incorrectly paired nucleotides from the newly synthesized strand and use the original nucleotide strand as a template when replacing them.
missense mutation
Alters a codon in mRNA, resulting in a different amino acid in the protein encoded.
mitochondrial DNA (mtDNA)
DNA in mitochondria; has some characteristics in common with eubacterial DNA and typically consists of a circular molecule that lacks histone proteins and encodes some of the rRNAs, tRNAs, and proteins found in mitochondria.
mitosis
Process by which the nucleus of a eukaryotic cell divides.
mitosis-promoting factor (MPF)
Protein functioning in the control of the cell cycle; consists of a cyclin combined with cyclin-dependent kinase (CDK). Active MPF stimulates mitosis.
mitotic spindle
Array of microtubules that radiate from two poles; moves chromosomes in mitosis and meiosis.
model genetic organism
An organism that is widely used in genetic studies because it has characteristics, such as short generation time and large numbers of progeny, that make it particularly useful for genetic analysis.
moderately repetitive DNA
DNA consisting of sequences that are from 150 to 300 bp in length and are repeated thousands of times.
modified base
Rare base found in some RNA molecules. Such bases are modified forms of the standard bases (adenine, guanine, cytosine, and uracil).
molecular chaperone
Molecule that assists in the proper folding of another molecule.
molecular clock
Refers to the use of molecular differences to estimate the time of divergence between organisms; assumes a roughly constant rate at which one neutral mutation replaces another.
molecular genetics
Study of the chemical nature of genetic information and how it is encoded, replicated, and expressed.
molecular motor
Specialized protein that moves cellular components.
monoecious organism
Individual organism that has both male and female reproductive structures.
monohybrid cross
A cross between two individuals that differ in a single characteristic—more specifically, a cross between individuals that are homozygous for different alleles at the same locus (AA × aa); also refers to a cross between two individuals that are heterozygous for two alleles at a single locus (Aa × Aa).
monosomy
Absence of one of the chromosomes of a homologous pair.
monozygotic twins
Identical twins that arise when a single egg fertilized by a single sperm splits into two separate embryos.
morphogen
Molecule whose concentration gradient affects the developmental fate of surrounding cells.
mosaicism
Condition in which regions of tissue within a single individual have different chromosome constitutions.
M (mitotic) phase
Period of active cell division; includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).
mRNA surveillance
Mechanisms for the detection and elimination of mRNAs that contain errors that may create problems in the course of translation.
multifactorial characteristic
Determined by multiple genes and environmental factors.
multigene family
Set of genes similar in sequence that arose through repeated duplication events; often encode different proteins.
multiple alleles
Presence in a group of individuals of more than two alleles at a locus. However, each member of the group has only two of the possible alleles.
multiple 3′ cleavage sites
Refers to the presence of more than one 3′ cleavage site on a single pre-mRNA, which allows cleavage and polyadenylation to take place at different sites, producing mRNAs of different lengths.
multiplication rule
States that the probability of two or more independent events occurring together is calculated by multiplying the probabilities of each of the individual events.
mutagen
Any environmental agent that significantly increases the rate of mutation above the spontaneous rate.
mutagenesis screen
Method for identifying genes that influence a specific phenotype. Random mutations are induced in a population of organisms, and individual organisms with mutant phenotypes are identified. These individual organisms are crossed to determine the genetic basis of the phenotype and to map the location of mutations that cause the phenotype.
mutation
Heritable change in genetic information.
mutation driver
A mutation found in a cancer cell that contributes to the process of cancer development.
mutation passenger
A mutation found in a cancer cell that does not contribute to the development of cancer.
mutation rate
Frequency with which a gene changes from the wild type to a specific mutant; generally expressed as the number of mutations per biological unit (that is, mutations per cell division, per gamete, or per round of replication).
narrow-sense heritability
Proportion of the phenotypic variance that can be attributed to additive genetic variance.
natural selection
Differential reproduction of genotypes.
negative assortative mating
Mating between unlike individuals that is more frequent than would be expected on the basis of chance.
negative control
Gene regulation in which the binding of a regulatory protein to DNA inhibits transcription (the regulatory protein is a repressor).
negative supercoiling
See supercoiling.
neutral mutation
Changes the amino acid sequence of a protein but does not alter the function of the protein.
neutral-mutation hypothesis
Proposes that much of the molecular variation seen in natural populations is adaptively neutral and unaffected by natural selection. Under this hypothesis, individuals with different molecular variants have equal fitnesses.
newborn screening
Tests newborn infants for certain genetic disorders.
next-generation sequencing technologies
Sequencing methods, such as pyrosequencing, that are capable of simultaneously determining the sequences of many DNA fragments; the technologies are much faster and less expensive than the Sanger method.
nitrogenous base
Nitrogen-containing base that is one of the three parts of a nucleotide.
node
Point in a phylogenetic tree that represents an organism. Terminal nodes are those that are at the outermost branches of the tree and represent organisms for which data have been obtained. Internal nodes represent ancestors common to organisms on different branches of the tree.
no-go decay
A mRNA surveillance system in eukaryotes that helps remove stalled ribosomes resulting from secondary structures in mRNA, chemical damage to mRNA, premature stop codons, and ribosomal defects.
nonautonomous element
Transposable element that cannot transpose on its own but can transpose in the presence of an autonomous element of the same family.
nondisjunction
Failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis.
nonhistone chromosomal protein
One of a heterogeneous assortment of nonhistone proteins in chromatin.
nonidentical twins
Twins that arise when two different eggs are fertilized by two different sperm; also called dizygotic twins or fraternal twins.
noninvasive prenatal genetic diagnosis
A genetic test performed on an unborn fetus without taking a tissue sample from the fetus. Usually performed by testing fetal DNA that occurs within the maternal blood.
nonoverlapping genetic code
Refers to the fact that, generally, each nucleotide is a part of only one codon and encodes only one amino acid in a protein.
nonreciprocal translocation
Movement of a chromosome segment to a nonhomologous chromosome or region without any (or with unequal) reciprocal exchange of segments.
nonrecombinant (parental) gamete
Contains only the original combinations of genes present in the parents.
nonrecombinant (parental) progeny
Possesses the original combinations of traits possessed by the parents.
nonreplicative transposition
Type of transposition in which a transposable element excises from an old site and moves to a new site, resulting in no net increase in the number of copies of the transposable element.
nonsense codon
Codon in mRNA that signals the end of translation; also called a stop codon or termination codon. The three common nonsense codons are UAA, UAG, and UGA.
nonsense-mediated mRNA decay (NMD)
Process that brings about the rapid elimination of mRNA that has a premature stop codon.
nonsense mutation
Changes a sense codon (one that specifies an amino acid) into a stop codon.
nonstop RNA decay
Mechanism in eukaryotic cells for dealing with ribosomes stalled at the 3′ end of an mRNA that lacks a termination codon. A protein binds to the A site of the stalled ribosome and recruits other proteins that degrade the mRNA from the 3′ end.
nontemplate strand
The DNA strand that is complementary to the template strand; not ordinarily used as a template during transcription.
normal distribution
Common type of frequency distribution that exhibits a symmetrical, bell-shaped curve; usually arises when a large number of independent factors contribute to the measurement.
Northern blotting
Process by which RNA is transferred from a gel to a solid support such as a nitrocellulose or nylon filter.
nuclear envelope
Membrane that surrounds the genetic material in eukaryotic cells to form a nucleus; segregates the DNA from other cellular contents.
nuclear matrix
Network of protein fibers in the nucleus; holds the nuclear contents in place.
nuclear pre-mRNA intron
Belongs to a class of introns in protein-encoding genes that reside in the nuclei of eukaryotic cells; removed by spliceosomal-mediated splicing.
nucleoid
Bacterial DNA confined to a definite region of the cytoplasm.
nucleoside
Ribose or deoxyribose bonded to a base.
nucleosome
Basic repeating unit of chromatin, consisting of a core of eight histone proteins (two each of H2A, H2B, H3, and H4) and about 146 bp of DNA that wraps around the core about two times.
nucleotide
Repeating unit of DNA or RNA made up of a sugar, a phosphate, and a base.
nucleotide-excision repair
DNA repair that removes bulky DNA lesions and other types of DNA damage.
nucleus
Space in eukaryotic cells that is enclosed by the nuclear envelope and contains the chromosomes.
nullisomy
Absence of both chromosomes of a homologous pair (2n − 2).
Okazaki fragment
Short stretch of newly synthesized DNA. Produced by discontinuous replication on the lagging strand, these fragments are eventually joined together.
oligonucleotide-directed mutagenesis
Method of site-directed mutagenesis that uses an oligonucleotide to introduce a mutant sequence into a DNA molecule.
oncogene
Dominant-acting gene that stimulates cell division, leading to the formation of tumors and contributing to cancer; arises from mutated copies of a normal cellular gene (proto-oncogene).
one gene, one enzyme hypothesis
Idea proposed by Beadle and Tatum that each gene encodes a separate enzyme.
one gene, one polypeptide hypothesis
Modification of the one gene, one enzyme hypothesis; proposes that each gene encodes a separate polypeptide chain.
oogenesis
Egg production in animals.
oogonium
Diploid cell in the ovary; capable of undergoing meiosis to produce an egg cell.
open reading frame (ORF)
Continuous sequence of DNA nucleotides that contains a start codon and a stop codon in the same reading frame; is assumed to be a gene that encodes a protein but, in many cases, the protein has not yet been identified.
operator
DNA sequence in the operon of a bacterial cell. A regulator protein binds to the operator and affects the rate of transcription of structural genes.
operon
Set of structural genes in a bacterial cell along with a common promoter and other sequences (such as an operator) that control the transcription of the structural genes.
origin of replication
Site where DNA synthesis is initiated.
orthologous genes
Homologous genes found in different species because the two species have a common ancestor that also possessed the gene.
outcrossing
Mating between unrelated individuals that is more frequent than would be expected on the basis of chance.
overdominance
Selection in which the heterozygote has higher fitness than that of either homozygote; also called heterozygote advantage.
ovum
Final product of oogenesis.
pachytene
Third substage of prophase I in meiosis. The synaptonemal complex forms during pachytene.
pair-rule genes
Set of segmentation genes in fruit flies that define regional sections of the embryo and affect alternate segments. Mutations in these genes often cause the deletion of every other segment.
palindrome
Sequence of nucleotides that reads the same on complementary strands; inverted repeats.
pangenesis
Early concept of heredity proposing that particles carry genetic information from different parts of the body to the reproductive organs.
paracentric inversion
Chromosome inversion that does not include the centromere in the inverted region.
paralogous genes
Homologous genes in the same species that arose through the duplication of a single ancestral gene.
paramutation
Epigenetic change in which one allele of a genotype alters the expression of another allele; the altered expression persists for several generations, even after the altering allele is no longer present.
parental gamete
See nonrecombinant (parental) gamete.
parental progeny
See nonrecombinant (parental) progeny.
partial diploid
Bacterial cell that possesses two copies of genes, including one copy on the bacterial chromosome and the other on an extra piece of DNA (usually a plasmid); also called merozygote.
Patau syndrome (trisomy 13)
Characterized by severe intellectual disability, a small head, sloping forehead, small eyes, cleft lip and palate, extra fingers and toes, and other disabilities; results from the presence of three copies of chromosome 13.
pedigree
Pictorial representation of a family history outlining the inheritance of one or more traits or diseases.
penetrance
Percentage of individuals with a particular genotype that express the phenotype expected of that genotype.
pentaploidy
Possession of five haploid sets of chromosomes (5n).
peptide bond
Chemical bond that connects amino acids in a protein.
peptidyl (P) site
One of three sites in a ribosome occupied by a tRNA in translation. In the elongation stage of protein synthesis, tRNAs move from the aminoacyl (A) site into the P site.
peptidyl transferase
Activity in the ribosome that creates a peptide bond between two amino acids. Evidence suggests that this activity is carried out by one of the RNA components of the ribosome.
pericentric inversion
Chromosome inversion that includes the centromere in the inverted region.
P (parental) generation
First set of parents in a genetic cross.
phenocopy
Phenotype that is produced by environmental effects and is the same as the phenotype produced by a genotype.
phenotype
Appearance or manifestation of a characteristic.
phenotypic correlation
Correlation between two or more phenotypes in the same individual.
phenotypic variance
Measures the degree of phenotypic differences among a group of individuals; composed of genetic, environmental, and genetic–environmental interaction variances.
phenylketonuria (PKU)
Genetic disease characterized by intellectual disability, light skin, and eczema; caused by mutations in the gene that encodes phenylalanine hydroxylase (PAH), a liver enzyme that normally metabolizes the amino acid phenylalanine. When the enzyme is defective, phenylalanine is not metabolized and builds up to high levels in the body, eventually causing intellectual disability and other characteristics of the disease. The disease is inherited as an autosomal recessive disorder and can be effectively treated by limiting phenylalanine in the diet.
phosphate group
A phosphorus atom attached to four oxygen atoms; one of the three components of a nucleotide.
phosphodiester
Molecule containing R–O–P–O–R, where R is a carbon-containing group, O is oxygen, and P is phosphorus.
phosphodiester linkage
Phosphodiester bond connecting two nucleotides in a polynucleotide strand.
phylogenetic tree
Graphical representation of the evolutionary connections between organisms or genes.
phylogeny
Evolutionary relationships among a group of organisms or genes, usually depicted as a family tree or branching diagram.
physical map
Map of physical distances between loci, genetic markers, or other chromosome segments; measured in base pairs.
pilus (pl., pili)
Extension of the surface of some bacteria that allows conjugation to take place. When a pilus on one cell makes contact with a receptor on another cell, the pilus contracts and pulls the two cells together.
Piwi-interacting RNA (piRNA)
Small RNA molecule belonging to a class named after Piwi proteins with which these molecules interact; similar to microRNA and small interfering RNA and thought to have a role in the regulation of sperm development.
plaque
Clear patch of lysed cells on a continuous layer of bacteria on the agar surface of a petri plate. Each plaque represents a single original phage that multiplied and lysed many cells.
plasmid
Small, circular DNA molecule found in bacterial cells that is capable of replicating independently from the bacterial chromosome.
pleiotropy
A single genotype influences multiple phenotypes.
pluripotency
In embryonic stem cells, the property of being undifferentiated, with the capacity to form every type of cell in an organism.
poly(A)-binding protein (PABP)
Binds to the poly(A) tail of eukaryotic mRNA and makes the mRNA more stable. There are several types of PABP.
poly(A) tail
String of adenine nucleotides added to the 3′ end of a eukaryotic mRNA after transcription.
polycistronic mRNA
Single bacterial RNA molecule that encodes more than one polypeptide chain; uncommon in eukaryotes.
polygenic characteristic
Encoded by genes at many loci.
polymerase chain reaction (PCR)
Method of enzymatically amplifying DNA fragments.
polynucleotide strand
Series of nucleotides linked together by phosphodiester bonds.
polypeptide
Chain of amino acids linked by peptide bonds; also called a protein.
polyploidy
Possession of more than two haploid sets of chromosomes.
polyribosome
Messenger RNA molecule with several ribosomes attached to it.
polytene chromosome
Giant chromosome in the salivary glands of Drosophila melanogaster. Each polytene chromosome consists of a number of DNA molecules lying side by side.
population
The group of interest; often represented by a subset called a sample. Also, a group of members of the same species.
population genetics
Study of the genetic composition of populations (groups of members of the same species) and how a population’s collective group of genes changes with the passage of time.
positional cloning
Method that allows for the isolation and identification of a gene by examining the cosegregation of a phenotype with previously mapped genetic markers.
position effect
Dependence of the expression of a gene on the gene’s location in the genome.
positive assortative mating
Mating between like individuals that is more frequent than would be expected on the basis of chance.
positive control
Gene regulation in which the binding of a regulatory protein to DNA stimulates transcription (the regulatory protein is an activator).
positive supercoiling
See supercoiling.
posttranslational modification
Alteration of a protein after translation; may include cleavage from a larger precursor protein, the removal of amino acids, and the attachment of other molecules to the protein.
postzygotic reproductive isolating mechanism
Reproductive isolation that arises after a zygote is formed, either because the resulting hybrids are inviable or sterile or because reproduction breaks down in subsequent generations.
preformationism
Early concept of inheritance proposing that a miniature adult (homunculus) resides in either the egg or the sperm and increases in size in development, with all traits being inherited from the parent that contributes the homunculus.
preimplantation genetic diagnosis (PGD)
Genetic testing on an embryo produced by in vitro fertilization before implantation of the embryo in the uterus.
pre-messenger RNA (pre-mRNA)
Eukaryotic RNA molecule that is modified after transcription to become mRNA.
presymptomatic genetic testing
Tests people to determine whether they have inherited a disease-causing gene before the symptoms of the disease have appeared.
prezygotic reproductive isolating mechanism
Reproductive isolation in which gametes from two different species are prevented from fusing and forming a hybrid zygote.
primary Down syndrome
Caused by the presence of three copies of chromosome 21.
primary immune response
Initial clone of cells specific for a particular antigen and generated when the antigen is first encountered by the immune system.
primary oocyte
Oogonium that has entered prophase I.
primary spermatocyte
Spermatogonium that has entered prophase I.
primary structure of a protein
The amino acid sequence of a protein.
primase
Enzyme that synthesizes a short stretch of RNA on a DNA template; functions in replication to provide a 3′-OH group for the attachment of a DNA nucleotide.
primer
Short stretch of RNA on a DNA template; provides a 3′-OH group for the attachment of a DNA nucleotide at the initiation of replication.
principle of independent assortment (Mendel’s second law)
Important principle of heredity discovered by Mendel that states that genes encoding different characteristics (genes at different loci) separate independently; applies only to genes located on different chromosomes or to genes far apart on the same chromosome.
principle of segregation (Mendel’s first law)
Important principle of heredity discovered by Mendel that states that each diploid individual possesses two alleles at a locus and that these two alleles separate when gametes are formed, one allele going into each gamete.
probability
Likelihood of the occurrence of a particular event; more formally, the number of times that a particular event occurs divided by the number of all possible outcomes. Probability values range from 0 to 1.
proband
A person having a trait or disease for whom a pedigree is constructed.
probe
Known sequence of DNA or RNA that is complementary to a sequence of interest and will pair with it; used to find specific DNA sequences.
prokaryote
Unicellular organism with a simple cell structure. Prokaryotes include eubacteria and archaea.
prometaphase
Stage of mitosis. In prometaphase, the nuclear membrane breaks down and the spindle microtubules attach to the chromosomes.
promoter
DNA sequence to which the transcription apparatus binds so as to initiate transcription; indicates the direction of transcription, which of the two DNA strands is to be read as the template, and the starting point of transcription.
proofreading
Ability of DNA polymerases to remove and replace incorrectly paired nucleotides in the course of replication.
prophage
Phage genome that is integrated into a bacterial chromosome.
prophase
Stage of mitosis. In prophase, the chromosomes contract and become visible, the cytoskeleton breaks down, and the mitotic spindle begins to form.
prophase I
Stage of meiosis I. In prophase I, chromosomes condense and pair, crossing over takes place, the nuclear membrane breaks down, and the spindle forms.
prophase II
Stage of meiosis after interkinesis. In prophase II, chromosomes condense, the nuclear membrane breaks down, and the spindle forms. Some cells skip this stage.
protein-coding region
The part of mRNA consisting of the nucleotides that specify the amino acid sequence of a protein.
protein domain
Region of a protein that has a specific shape or function.
protein kinase
Enzyme that adds phosphate groups to other proteins.
protein microarray
Large number of different proteins applied to a glass slide as a series of spots, each spot containing a different protein; used to analyze protein-protein interactions.
proteome
Set of all proteins encoded by a genome.
proteomics
Study of the proteome, the complete set of proteins found in a given cell.
proto-oncogene
Normal cellular gene that controls cell division. When mutated, it may become an oncogene and contribute to cancer progression.
prototrophic bacterium
A wild-type bacterium that can use a carbon source, essential elements such as nitrogen and phosphorus, certain vitamins, and other required ions and nutrients to synthesize all the compounds that they need for growth and reproduction.
provirus
DNA copy of viral DNA or viral RNA; integrated into the host chromosome and replicated along with the host chromosome.
pseudoautosomal region
Small region of the X and Y chromosomes that contains homologous gene sequences.
pseudodominance
Expression of a normally recessive allele owing to a deletion on the homologous chromosome.
Punnett square
Shorthand method of determining the outcome of a genetic cross. On a grid, the gametes of one parent are written along the upper edge and the gametes of the other parent are written along the left-hand edge. Within the cells of the grid, the alleles in the gametes are combined to form the genotypes of the offspring.
purine
Type of nitrogenous base in DNA and RNA. Adenine and guanine are purines.
pyrimidine
Type of nitrogenous base in DNA and RNA. Cytosine, thymine, and uracil are pyrimidines.
pyrimidine dimer
Structure in which a bond forms between two adjacent pyrimidine molecules on the same strand of DNA; disrupts normal hydrogen bonding between complementary bases and distorts the normal configuration of the DNA molecule.
quantitative characteristic
Continuous characteristic; displays a large number of possible phenotypes, which must be described by a quantitative measurement.
quantitative genetics
Genetic analysis of complex characteristics or characteristics influenced by multiple genetic factors.
quantitative trait locus (QTL)
A gene or chromosomal region that contributes to the expression of quantitative characteristics.
quaternary structure of a protein
Interaction of two or more polypeptides to form a functional protein.
reading frame
Particular way in which a nucleotide sequence is read in groups of three nucleotides (codons) in translation. Each reading frame begins with a start codon and ends with a stop codon.
realized heritability
Narrow-sense heritability measured from a response-to-selection experiment.
real-time PCR
Modification of the polymerase chain reaction that quantitatively determines the amount of starting nucleic acid; the amount of DNA amplified is measured as the reaction proceeds.
reannealing
See renaturation.
recessive
Refers to an allele or phenotype that is expressed only when the recessive allele is present in two copies (homozygous). The recessive allele is not expressed in the heterozygote phenotype.
reciprocal crosses
Crosses in which the phenotypes of the male and female parents are reversed. For example, in one cross, a tall male is crossed with a short female and, in the other cross, a short male is crossed with a tall female.
reciprocal translocation
Reciprocal exchange of segments between two nonhomologous chromosomes.
recombinant DNA technology
Set of molecular techniques for locating, isolating, altering, combining, and studying DNA segments.
recombinant gamete
Possesses new combinations of genes.
recombinant progeny
Possesses new combinations of traits formed from recombinant gametes.
recombination
Process that produces new combinations of alleles.
recombination frequency
Proportion of recombinant progeny produced in a cross.
regression
Analysis of how one variable changes in response to another variable.
regression coefficient
Statistic that measures how much one variable changes, on average, with a unit change in another variable.
regulator gene
Gene associated with an operon in bacterial cells that encodes a protein or RNA molecule that functions in controlling the transcription of one or more structural genes.
regulator protein
Produced by a regulator gene, a protein that binds to another DNA sequence and controls the transcription of one or more structural genes.
regulatory element
DNA sequence that affects the transcription of other DNA sequences to which it is physically linked.
regulatory gene
DNA sequence that encodes a protein or RNA molecule that interacts with DNA sequences and affects their transcription or translation or both.
regulatory promoter
DNA sequence located immediately upstream of the core promoter that affects transcription; contains consensus sequences to which transcriptional regulator proteins bind.
relaxed state of DNA
Energy state of a DNA molecule when there is no structural strain on the molecule.
release factor
Protein required for the termination of translation; binds to a ribosome when a stop codon is reached and stimulates the release of the polypeptide chain, the tRNA, and the mRNA from the ribosome. Eukaryotic cells require two release factors (eRF-1 and eRF-2), whereas E. coli requires three (RF-1, RF-2, and RF-3).
renaturation
The process by which two complementary single-stranded DNA molecules pair; also called reannealing.
repetitive DNA
Sequences that exist in multiple copies in a genome.
replicated error
Replication of an incorporated error in which a change in the DNA sequence has been replicated and all base pairings in the new DNA molecule are correct.
replication
Process by which DNA is synthesized from a single-stranded nucleotide template.
replication bubble
Segment of a DNA molecule that is unwinding and undergoing replication.
replication fork
Point at which a double-stranded DNA molecule separates into two single strands that serve as templates for replication.
replication licensing factor
Protein that ensures that replication takes place only once at each origin; required at the origin before replication can be initiated and removed after the DNA has been replicated.
replication origin
Sequence of nucleotides where replication is initiated.
replication terminus
Point at which replication stops.
replicative segregation
Random segregation of organelles into progeny cells in cell division. If two or more versions of an organelle are present in the original cell, chance determines the proportion of each type that will segregate into each progeny cell.
replicative transposition
Type of transposition in which a copy of the transposable element moves to a new site while the original copy remains at the old site; increases the number of copies of the transposable element.
replicon
Unit of replication, consisting of DNA from the origin of replication to the point at which replication on either side of the origin ends.
repressible operon
Operon or other system of gene regulation in which transcription is normally on. Something must take place for transcription to be repressed, or turned off.
repressor
Regulatory protein that binds to a DNA sequence and inhibits transcription.
reproductive isolating mechanism
Any biological factor or mechanism that prevents gene exchange.
repulsion
See trans configuration.
resolvase
Enzyme required for some types of transposition; brings about resolution—that is, crossing over between sites located within the transposable element. Resolvase may be encoded by the transposable element or by a cellular enzyme that normally functions in homologous recombination.
response element
Common DNA sequence found upstream of some groups of eukaryotic genes. A regulatory protein binds to a response element and stimulates the transcription of a gene. The presence of the same response element in several promoters or enhancers allows a single factor to simultaneously stimulate the transcription of several genes.
response to selection
The amount that a characteristic changes in one generation owing to selection; equals the selection differential times the narrow-sense heritability.
restriction endonuclease
Technical term for a restriction enzyme, which recognizes particular base sequences in DNA and makes double-stranded cuts nearby.
restriction enzyme
Recognizes particular base sequences in DNA and makes double-stranded cuts nearby; also called restriction endonuclease.
restriction fragment length polymorphism (RFLP)
Variation in the pattern of fragments produced when DNA molecules are cut with the same restriction enzyme; represents a heritable difference in DNA sequences and can be used in genetic analyses.
restriction mapping
Determines the locations of sites cut by restriction enzymes in a piece of DNA.
retrotransposon
Type of transposable element in eukaryotic cells that possesses some characteristics of retroviruses and transposes through an RNA intermediate.
retrovirus
RNA virus capable of integrating its genetic material into the genome of its host. The virus injects its RNA genome into the host cell, where reverse transcription produces a complementary, double-stranded DNA molecule from the RNA template. The DNA copy then integrates into the host chromosome to form a provirus.
reverse duplication
Duplication of a chromosome segment in which the sequence of the duplicated segment is inverted relative to the sequence of the original segment.
reverse genetics
A molecular approach that begins with a genotype (a DNA sequence) and proceeds to the phenotype by altering the sequence or by inhibiting its expression.
reverse mutation (reversion)
Mutation that changes a mutant phenotype back into the wild type.
reverse transcriptase
Enzyme capable of synthesizing complementary DNA from an RNA template.
reverse transcription
Synthesis of DNA from an RNA template.
reverse-transcription PCR
Amplifies sequences corresponding to RNA. Reverse transcriptase is used to convert RNA into complementary DNA, which can then be amplified by the usual polymerase chain reaction.
rho-dependent terminator
Sequence in bacterial DNA that requires the presence of the rho subunit of RNA polymerase to terminate transcription.
rho factor
Subunit of bacterial RNA polymerase that facilitates the termination of transcription of some genes.
rho-independent terminator
Sequence in bacterial DNA that does not require the presence of the rho subunit of RNA polymerase to terminate transcription.
ribonucleoside triphosphate (rNTP)
Substrate of RNA synthesis; consists of ribose, a nitrogenous base, and three phosphates linked to the 5′-carbon atom of the ribose. In transcription, two of the phosphates are cleaved, producing an RNA nucleotide.
ribonucleotide
Nucleotide containing ribose; present in RNA.
ribose
Five-carbon sugar in RNA.
ribosomal RNA (rRNA)
RNA molecule that is a structural component of the ribosome.
riboswitch
Regulatory sequences in an RNA molecule. When an inducer molecule binds to the riboswitch, the binding changes the configuration of the RNA molecule and alters the expression of the RNA, usually by affecting the termination of transcription or affecting translation.
ribozyme
RNA molecule that can act as a biological catalyst.
RNA-coding region
Sequence of DNA nucleotides that encodes an RNA molecule.
RNA editing
Process in which the protein-coding sequence of an mRNA is altered after transcription. The amino acids specified by the altered mRNA are different from those predicted from the nucleotide sequence of the gene encoding the protein.
RNA-induced silencing complex (RISC)
Combination of a small interfering RNA (siRNA) molecule or a microRNA (miRNA) molecule and proteins that can cleave mRNA, leading to the degradation of the mRNA, or affecting transcription, or repressing translation of the mRNA.
RNA interference (RNAi)
Process in which cleavage of double-stranded RNA produces small interfering RNAs (siRNAs) that bind to mRNAs containing complementary sequences and bring about their cleavage and degradation.
RNA polymerase
Enzyme that synthesizes RNA from a DNA template during transcription.
RNA polymerase I
Eukaryotic RNA polymerase that transcribes large ribosomal RNA molecules (18 S rRNA and 28 S rRNA).
RNA polymerase II
Eukaryotic RNA polymerase that transcribes pre-messenger RNA, some small nuclear RNAs, and some microRNAs.
RNA polymerase III
Eukaryotic RNA polymerase that transcribes transfer RNA, small ribosomal RNAs (5 S rRNA), some small nuclear RNAs, and some microRNAs.
RNA polymerase IV
Transcribes small interfering RNAs in plants.
RNA polymerase V
Transcribes RNA that has a role in heterochromatin formation in plants.
RNA replication
Process in some viruses by which RNA is synthesized from an RNA template.
RNA silencing
Mechanism by which double-stranded RNA is cleaved and processed to yield small single-stranded interfering RNAs (siRNAs), which bind to complementary sequences in mRNA and bring about the cleavage and degradation of mRNA; also known as RNA interference and posttranscriptional RNA gene silencing. Some siRNAs also bind to complementary sequences in DNA and guide enzymes to methylate the DNA.
RNA splicing
Process by which introns are removed and exons are joined together.
Robertsonian translocation
Translocation in which the long arms of two acrocentric chromosomes become joined to a common centromere, resulting in a chromosome with two long arms and usually another chromosome with two short arms.
rolling-circle replication
Replication of circular DNA that is initiated by a break in one of the nucleotide strands, producing a double-stranded circular DNA molecule and a single-stranded linear DNA molecule, the latter of which may circularize and serve as a template for the synthesis of a complementary strand.
rooted tree
Phylogenetic tree in which one internal node represents the common ancestor of all other organisms (nodes) on the tree. In a rooted tree, all the organisms depicted have a common ancestor.
R plasmid (R factor)
Plasmid having genes that confer antibiotic resistance to any cell that contains the plasmid.
sample
Subset used to describe a population.
sampling error
Deviations from expected ratios due to chance occurrences when the number of events is small.
secondary immune response
Clone of cells generated when a memory cell encounters an antigen; provides long-lasting immunity.
secondary oocyte
One of the products of meiosis I in female animals; receives most of the cytoplasm.
secondary spermatocyte
Product of meiosis I in male animals.
secondary structure of a protein
Regular folding arrangement of amino acids in a protein. Common secondary structures found in proteins include the alpha helix and the beta pleated sheet.
second polar body
One of the products of meiosis II in oogenesis; contains a set of chromosomes but little of the cytoplasm.
segmental duplications
Regions larger than 1000 bp that are almost identical in sequence in eukaryotic genomes.
segmentation genes
Set of about 25 genes in fruit flies that control the differentiation of the embryo into individual segments, affecting the number and organization of the segments. Mutations in these genes usually disrupt whole sets of segments.
segment-polarity genes
Set of segmentation genes in fruit flies that affect the organization of segments. Mutations in these genes cause part of each segment to be deleted and replaced by a mirror image of part or all of an adjacent segment.
selection coefficient
Measure of the relative intensity of selection against a genotype; equals 1 minus fitness.
selection differential
Difference in phenotype between the selected individuals and the average of the entire population.
semiconservative replication
Replication in which the two nucleotide strands of DNA separate, each serving as a template for the synthesis of a new strand. All DNA replication is semiconservative.
sense codon
Codon that specifies an amino acid in a protein.
separase
Molecule that cleaves cohesin molecules, which hold the sister chromatids together.
sequence tagged site (STS)
Short (200-500 bp) sequence of DNA, present only once in the genome, whose chromosomal location has been determined. Often used to determine the genomic location of a DNA clone.
sequential hermaphroditism
Phenomenon in which the sex of an individual organism changes in the course of its lifetime; the organism is male at one age or developmental stage and female at a different age or stage.
70S initiation complex
Final complex formed in the initiation of translation in bacterial cells; consists of the small and large subunits of the ribosome, mRNA, and initiator tRNA charged with fMet.
sex chromosomes
Chromosomes that differ morphologically or in number in males and females.
sex determination
Specification of sex (male or female). Sex-determining mechanisms include chromosomal, genic, and environmental sex-determining systems.
sex-determining region Y (SRY) gene
On the Y chromosome, a gene that triggers male development.
sex-influenced characteristic
Characteristic encoded by autosomal genes that are more readily expressed in one sex. For example, an autosomal dominant gene may have higher penetrance in males than in females or an autosomal gene may be dominant in males but recessive in females.
sex-limited characteristic
Characteristic encoded by autosomal genes and expressed in only one sex. Both males and females carry genes for sex-limited characteristics, but the characteristics appear in only one of the sexes.
sex-linked characteristic
Characteristic determined by a gene or genes on sex chromosomes.
shelterin
A multiprotein complex that binds to mammalian telomeres and protects the ends of the DNA from being inadvertently repaired as a double-stranded break in the DNA.
Shine–Dalgarno sequence
Consensus sequence found in the bacterial 5′ untranslated region of mRNA; contains the ribosome-binding site.
short interspersed element (SINE)
Short DNA sequence repeated many times and interspersed throughout the genome.
short tandem repeat (STR)
Very short DNA sequence repeated in tandem and found widely in the human genome.
shuttle vector
Cloning vector that allows DNA to be transferred to more than one type of host cell.
sigma factor
Subunit of bacterial RNA polymerase that allows the RNA polymerase to recognize a promoter and initiate transcription.
signal sequence
From 15 to 30 amino acids that are found at the amino end of some eukaryotic proteins and direct the protein to specific locations in the cell; usually cleaved from the protein.
signal-transduction pathway
System in which an external signal (initiated by a hormone or growth factor) triggers a cascade of intracellular reactions that ultimately produce a specific response.
silencer
Sequence that has many of the properties possessed by an enhancer but represses transcription.
silent mutation
Change in the nucleotide sequence of DNA that does not alter the amino acid sequence of a protein.
single-nucleotide polymorphism (SNP)
Single-base-pair differences in DNA sequence between individual members of a species.
single-strand-binding (SSB) protein
Binds to single-stranded DNA in replication and prevents it from annealing with a complementary strand and forming secondary structures.
sister chromatids
Two copies of a chromosome that are held together at the centromere. Each chromatid consists of a single DNA molecule.
site-directed mutagenesis
Produces specific nucleotide changes at selected sites in a DNA molecule.
small cytoplasmic RNA (scRNA)
Small RNA molecule found in the cytoplasm of eukaryotic cells.
small interfering RNA (siRNA)
Single-stranded RNA molecule (usually from 21 to 25 nucleotides in length) produced by the cleavage and processing of double-stranded RNA; binds to complementary sequences in mRNA and brings about the cleavage and degradation of the mRNA. Some siRNAs bind to complementary sequences in DNA and bring about their methylation.
small nuclear ribonucleoprotein (snRNP)
Structure found in the nuclei of eukaryotic cells that consists of small nuclear RNA (snRNA) and protein; functions in the processing of pre-mRNA.
small nuclear RNA (snRNA)
Small RNA molecule found in the nuclei of eukaryotic cells; functions in the processing of pre-mRNA.
small nucleolar RNA (snoRNA)
Small RNA molecule found in the nuclei of eukaryotic cells; functions in the processing of rRNA and in the assembly of ribosomes.
small ribosomal subunit
The smaller of the two subunits of a functional ribosome.
somatic-cell hybridization
Fusion of different cell types.
somatic hypermutation
High rate of somatic mutation such as that in genes encoding antibodies.
somatic mutation
Mutation in a cell that does not give rise to a gamete.
somatic recombination
Recombination in somatic cells, such as maturing lymphocytes, among segments of genes that encode antibodies and T-cell receptors.
SOS system
System of proteins and enzymes that allow a cell to replicate its DNA in the presence of a distortion in DNA structure; makes numerous mistakes in replication and increases the rate of mutation.
Southern blotting
Process by which DNA is transferred from a gel to a solid support such as a nitrocellulose or nylon filter.
specialized transduction
Transduction in which genes near special sites on the bacterial chromosome are transferred from one bacterium to another; requires lysogenic bacteriophages.
speciation
Process by which new species arise. See also biological species concept, allopatric speciation, and sympatric speciation.
species
Term applied to different kinds or types of living organisms.
spermatid
Immediate product of meiosis II in spermatogenesis; matures to sperm.
spermatogenesis
Sperm production in animals.
spermatogonium
Diploid cell in the testis; capable of undergoing meiosis to produce a sperm.
S (synthesis) phase
Stage of interphase in the cell cycle. In S phase, DNA replicates.
spindle microtubule
Microtubule that moves chromosomes in mitosis and meiosis.
spindle pole
Point from which spindle microtubules radiate.
spliceosome
Large complex consisting of several RNAs and many proteins that splices protein-encoding pre-mRNA; contains five small ribonucleoprotein particles (U1, U2, U4, U5, and U6).
spontaneous mutation
Arises from natural changes in DNA structure or from errors in replication.
sporophyte
Diploid phase of the life cycle in plants.
SR proteins
Group of serine- and arginine-rich proteins that regulate alternative splicing of pre-mRNA.
standard deviation
Statistic that describes the variability of a group of measurements; the square root of the variance.
stem cells
Undifferentiated cells that are capable of forming every type of cell in an organism.
stop (termination or nonsense) codon
Codon in mRNA that signals the end of translation. The three common stop codons are UAA, UAG, and UGA.
strand slippage
Slipping of the template and newly synthesized strands in replication in which one of the strands loops out from the other and nucleotides are inserted or deleted on the newly synthesized strand.
structural gene
DNA sequence that encodes a protein that functions in metabolism or biosynthesis or that has a structural role in the cell.
structural genomics
Area of genomics that studies the organization and sequence of information contained within genomes; sometimes used by protein chemists to refer to the determination of the three-dimensional structure of proteins.
submetacentric chromsome
Chromosome in which the centromere is displaced toward one end, producing a short arm and a long arm.
supercoiling
Coiled tertiary structure that forms when strain is placed on a DNA helix by overwinding or underwinding of the helix. An overwound DNA exhibits positive supercoiling; an underwound DNA exhibits negative supercoiling.
suppressor mutation
Mutation that hides or suppresses the effect of another mutation at a site that is distinct from the site of the original mutation.
sympatric speciation
Speciation arising in the absence of any geographic barrier to gene flow; reproductive isolating mechanisms evolve within a single interbreeding population.
synapsis
Close pairing of homologous chromosomes.
synaptonemal complex
Three-part structure that develops between synapsed homologous chromosomes.
synonymous codons
Different codons that specify the same amino acid.
tag single-nucleotide polymorphism (tag-SNP)
Used to identify a haplotype.
tandem duplication
Chromosome rearrangement in which a duplicated chromosome segment is adjacent to the original segment.
tandem repeat sequences
DNA sequences repeated one after another; tend to be clustered at specific locations on a chromosome.
Taq polymerase
DNA polymerase commonly used in PCR reactions. Isolated from the bacterium Thermus aquaticus, the enzyme is stable at high temperatures, and so it is not denatured during the strand-separation step of the cycle.
TATA-binding protein (TBP)
Polypeptide chain found in several different transcription factors that recognizes and binds to sequences in eukaryotic promoters.
TATA box
Consensus sequence (TATAAAA) commonly found in eukaryotic RNA polymerase II promoters; usually located from 25 to 30 bp upstream of the transcription start site. The TATA box determines the start point for transcription.
T cell
Particular type of lymphocyte that produces cellular immunity; originates in the bone marrow and matures in the thymus.
T-cell receptor
Found on the surface of a T cell, a receptor that simultaneously binds a foreign and a self-antigen on the surface of a cell.
telocentric chromosome
Chromosome in which the centromere is at or very near one end.
telomerase
Enzyme that is made up of both protein and RNA and replicates the ends (telomeres) of eukaryotic chromosomes. The RNA part of the enzyme has a template that is complementary to repeated sequences in the telomere and pairs with them, providing a template for the synthesis of additional copies of the repeats.
telomere
Stable end of a chromosome.
telomere-associated sequence
Sequence found at the ends of a chromosome next to the telomeric sequence; consists of long, complex repeated sequences.
telomeric sequence
Sequence found at the ends of a chromosome; consists of many copies of short, simple sequences repeated one after the other.
telophase
Stage of mitosis. In telophase, the chromosomes arrive at the spindle poles, the nuclear membrane re-forms, and the chromosomes relax and lengthen.
telophase I
Stage of meiosis I. In telophase I, chromosomes arrive at the spindle poles.
telophase II
Stage of meiosis II. In telophase II, chromosomes arrive at the spindle poles.
temperate phage
Bacteriophage that utilizes the lysogenic cycle, in which the phage DNA integrates into the bacterial chromosome and remains in an inactive state.
temperature-sensitive allele
Expressed only at certain temperatures.
template strand
The strand of DNA that is used as a template during transcription. The RNA synthesized during transcription is complementary and antiparallel to the template strand.
temporal isolation
Reproductive isolation in which the reproduction of different groups takes place at different times of the year, and so there is no gene flow between groups; exemplified by species of plants that flower at different times of the year and thus do not exchange genes.
terminal inverted repeats
Sequences found at both ends of a transposable element that are inverted complements of one another.
termination codon
Codon in mRNA that signals the end of translation; also called nonsense codon or stop codon. The three common termination codons are UAA, UAG, and UGA.
terminator
Sequence of DNA nucleotides that causes the termination of transcription.
tertiary structure of a protein
Higher-order folding of amino acids in a protein to form the overall three-dimensional shape of the molecule.
testcross
A cross between an individual with an unknown genotype and an individual with the homozygous recessive genotype.
tetrad
The four products of meiosis; all four chromatids of a homologous pair of chromosomes.
tetrad analysis
Genetic analysis of a tetrad, the products of a single meiosis.
tetraploidy
Possession of four haploid sets of chromosomes (4n).
tetrasomy
Presence of two extra copies of a chromosome (2n + 2).
TFIIB recognition element (BRE)
Consensus sequence [(G or C)(G or C)(G or C)CGCC] found in some RNA polymerase II core promoters; usually located from 32 to 38 bp upstream of the transcription start site.
theory of clonal selection
Explains the generation of primary and secondary immune responses. The binding of a B cell to an antigen stimulates the cell to divide, giving rise to a clone of genetically identical cells, all of which are specific for the antigen.
theta replication
Replication of circular DNA that is initiated by the unwinding of the two nucleotide strands, producing a replication bubble. Unwinding continues at one or both ends of the bubble, making it progressively larger. DNA replication on both of the template strands is simultaneous with unwinding until the two replication forks meet.
30S initiation complex
Initial complex formed in the initiation of translation in bacterial cells; consists of the small subunit of the ribosome, mRNA, initiator tRNA charged with fMet, GTP, and initiation factors 1, 2, and 3.
three-point testcross
Cross between an individual heterozygous at three loci and an individual homozygous for recessive alleles at those loci.
3′ end
End of a polynucleotide chain where an OH group is attached to the 3′-carbon atom of the nucleotide.
3′ splice site
The 3′ end of an intron where cleavage takes place in RNA splicing.
3′ untranslated (3′ UTR) region
Sequence of nucleotides at the 3′ end of mRNA; does not encode the amino acids of a protein but affects both the stability of the mRNA and its translation.
threshold characteristic
Discontinuous characteristic whose expression depends on an underlying susceptibility that varies continuously.
thymine (T)
Pyrimidine base in DNA but not in RNA.
Ti plasmid
Large plasmid isolated from the bacterium Agrobacterium tumefaciens and used to transfer genes to plant cells.
topoisomerase
Enzyme that adds or removes rotations in a DNA helix by temporarily breaking nucleotide strands; controls the degree of DNA supercoiling.
totipotency
The potential of a cell to develop into any other cell type.
trans configuration
Arrangement in which each chromosome contains one wild-type (dominant) gene and one mutant (recessive) gene; also called repulsion.
transcription
Process by which RNA is synthesized from a DNA template.
transcriptional activator protein
Protein in eukaryotic cells that binds to consensus sequences in regulatory promoters or enhancers and affects transcription initiation by stimulating the assembly of the basal transcription apparatus.
transcription bubble
Region of a DNA molecule that has unwound to expose a single-stranded template, which is being transcribed into RNA.
transcription factor
Protein that binds to DNA sequences in eukaryotic cells and affects transcription.
transcription start site
The first DNA nucleotide that is transcribed into an RNA molecule.
transcription unit
Sequence of nucleotides in DNA that encodes a single RNA molecule, along with the sequences necessary for its transcription; normally contains a promoter, an RNA-coding sequence, and a terminator.
transcriptome
Set of all RNA molecules transcribed from a genome.
transducing phage
Contains a piece of the bacterial chromosome inside the phage coat. See also generalized transduction.
transductant
Bacterial cell that has received genes from another bacterium through transduction.
transduction
Type of gene exchange that takes place when a virus carries genes from one bacterium to another. After it is inside the cell, the newly introduced DNA may undergo recombination with the bacterial chromosome.
transesterification
Chemical reaction in some RNA-splicing reactions.
transfer-messenger RNA (tmRNA)
An RNA molecule that has properties of both mRNA and tRNA; functions in rescuing ribosomes that are stalled at the end of mRNA.
transfer RNA (tRNA)
RNA molecule that carries an amino acid to the ribosome and transfers it to a growing polypeptide chain in translation.
transfer RNA intron
Belongs to a class of introns in tRNA genes. The splicing of these genes relies on enzymes.
transformant
Cell that has received genetic material through transformation.
transformation
Mechanism by which DNA found in the medium is taken up by the cell. After transformation, recombination may take place between the introduced genes and the cellular chromosome.
transforming principle
Substance responsible for transformation. DNA is the transforming principle.
transgene
Foreign gene or other DNA fragment carried in germline DNA.
transgenic mouse
Mouse whose genome contains a foreign gene or genes added by employing recombinant DNA methods.
transition
Base substitution in which a purine is replaced by a different purine or a pyrimidine is replaced by a different pyrimidine.
translation
Process by which a protein is assembled from information contained in messenger RNA.
translesion DNA polymerase
Specialized DNA polymerases that are able to replicate through distorted structures and bulky lesions that halt other DNA polymerases. They often make more errors during DNA synthesis than other DNA polymerases.
translocation
Movement of a chromosome segment to a nonhomologous chromosome or to a region within the same chromosome. Also, movement of a ribosome along mRNA in the course of translation.
translocation carrier
Individual organism heterozygous for a chromosome translocation.
transmission genetics
Field of genetics that encompasses the basic principles of genetics and how traits are inherited.
transposable element
DNA sequence capable of moving from one site to another within the genome through a mechanism that differs from that of homologous recombination.
transposase
Enzyme encoded by many types of transposable elements that is required for their transposition. The enzyme makes single-strand breaks at each end of the transposable element and on either side of the target sequence where the element inserts.
transposition
Movement of a transposable genetic element from one site to another. Replicative transposition increases the number of copies of the transposable element; nonreplicative transposition does not increase the number of copies.
trans-splicing
The process of splicing together exons from two or more pre-mRNAs.
transversion
Base substitution in which a purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine.
trihybrid cross
A cross between two individuals that differ in three characteristics (AA BB CC × aa bb cc); also refers to a cross between two individuals that are both heterozygous at three loci (Aa Bb Cc × Aa Bb Cc).
triplet code
Refers to the fact that three nucleotides encode each amino acid in a protein.
triploidy
Possession of three haploid sets of chromosomes (3n).
triplo-X syndrome
Human condition in which cells contain three X chromosomes. A person with triplo-X syndrome has a female phenotype without distinctive features other than a tendency to be tall and thin; a few such women are sterile, but many menstruate regularly and are fertile.
trisomy
Presence of an additional copy of a chromosome (2n + 1).
trisomy 8
Presence of three copies of chromosome 8; in humans, results in intellectual disability, contracted fingers and toes, low-set malformed ears, and a prominent forehead.
trisomy 13
Presence of three copies of chromosome 13; in humans, results in Patau syndrome.
trisomy 18
Presence of three copies of chromosome 18; in humans, results in Edward syndrome.
trisomy 21
Presence of three copies of chromosome 21; in humans, results in Down syndrome.
tRNA charging
Chemical reaction in which an aminoacyl-tRNA synthetase attaches an amino acid to its corresponding tRNA.
tRNA-modifying enzyme
Creates a modified base in RNA by catalyzing a chemical change in the standard base.
tubulin
Protein found in microtubules.
tumor-suppressor gene
Gene that normally inhibits cell division. Recessive mutations in such genes often contribute to cancer.
Turner syndrome
Human condition in which cells contain a single X chromosome and no Y chromosome (XO). Persons with Turner syndrome are female in appearance but do not undergo puberty and have poorly developed female secondary sex characteristics; most are sterile but have normal intelligence.
two-dimensional polyacrylamide gel electrophorsis (2D-PAGE)
Method for separating proteins into spots in which the proteins are separated in one dimension by charge, separated in a second dimension by mass, and then stained. Each spot is proportional to the amount of protein present.
two-point testcross
Cross between an individual heterozygous at two loci and an individual homozygous for recessive alleles at those loci.
ultrasonography
Procedure for visualizing the fetus. High-frequency sound is beamed into the uterus. Sound waves that encounter dense tissue bounce back and are transformed into a picture of the fetus.
unbalanced gamete
Gamete that has a variable number of chromosomes; some chromosomes may be missing and others may be present in more than one copy.
underdominance
Selection in which the heterozygote has lower fitness than that of either homozygote.
unequal crossing over
Misalignment of the two DNA molecules during crossing over, resulting in one DNA molecule with an insertion and the other with a deletion.
uniparental disomy
Inheritance of both chromosomes of a homologous pair from a single parent.
unique-sequence DNA
Sequence present only once or a few times in a genome.
universal genetic code
Refers to the fact that particular codons specify the same amino acids in almost all organisms.
up mutation
Mutation that increases the rate of transcription.
upstream element
Consensus sequence found in some bacterial promoters that contains a number of A-T pairs and is found about 40 to 60 bp upstream of the transcription start site.
uracil (U)
Pyrimidine base in RNA but not normally in DNA.
variable number of tandem repeats (VNTRs)
Short sequences repeated in tandem that vary greatly in number among individuals; also called microsatellites. Because they are quite variable, VNTRs are commonly used in DNA fingerprinting.
variance
Statistic that describes the variability of a group of measurements.
virulent phage
Bacteriophage that reproduces only through the lytic cycle and kills its host cell.
virus
Noncellular replicating agent consisting of nucleic acid surrounded by a protein coat; can replicate only within its host cell.
Western blotting
Process by which protein is transferred from a gel to a solid support such as a nitrocellulose or nylon filter.
whole-genome shotgun sequencing
Method of sequencing a genome in which sequenced fragments are assembled into the correct sequence in contigs by using only the overlaps in sequence.
wild type
The trait or allele that is most commonly found in natural (wild) populations.
wobble
Base pairing between codon and anticodon in which there is nonstandard pairing, usually at the third (3′) position of the codon; allows more than one codon to pair with the same anticodon.
X-linked characteristic
Characteristic determined by a gene or genes on the X chromosome.
X-ray diffraction
Method for analyzing the three-dimensional shape and structure of chemical substances. Crystals of a substance are bombarded with X-rays, which hit the crystals, bounce off, and produce a diffraction pattern on a detector. The pattern of the spots produced on the detector provides information about the molecular structure.
yeast artificial chromosome (YAC)
Cloning vector consisting of a DNA molecule with a yeast origin of replication, a pair of telomeres, and a centromere. YACs can carry very large pieces of DNA (as large as several hundred thousand base pairs) and replicate and segregate as yeast chromosomes do.
Y-linked characteristic
Characteristic determined by a gene or genes on the Y chromosome.
Z-DNA
Secondary structure of DNA characterized by 12 bases per turn, a left-handed helix, and a sugar-phosphate backbone that zigzags back and forth.
zygotene
Second substage of prophase I in meiosis. In zygotene, chromosomes enter into synapsis.