10-nm chromatin fiber
A relaxed 30-nm chromatin fiber, the state of the chromatin fiber in regions of the nucleus where transcription is currently taking place.
3′ end
The end of a nucleic acid strand that carries a free 3′ hydroxyl.
30-nm chromatin fiber
A chromosomal conformation created by the folding of the nucleosome fiber of DNA and histones.
3-phosphoglycerate (3-PGA)
A 3-carbon molecule; two molecules of 3-PGA are the first stable products of the Calvin cycle.
5′ cap
The modification of the 5′ end of the primary transcript by the addition of a special nucleotide attached in an unusual chemical linkage.
5′ end
The end of a nucleic acid strand containing a free 5′ phosphate group.
ABC model
A model of floral development that invokes three activities, A, B, and C, each of which represents the function of a protein or proteins hypothesized to be present in the cells of each whorl.
absolute temperature (T)
Temperature measured on the Kelvin scale.
accessory pigment
A pigment other than chlorophyll in the thylakoid membrane; carotenoids are important accessory pigments.
acidic
Describes a solution in which the concentration of protons is higher than that of hydroxide ions (the pH is lower than 7).
actin
A protein subunit that makes up microfilaments; used by both striated and smooth muscles to contract and generate force.
activated
The state of the receptor after binding the signaling molecule; the activated receptor transmits the information through the cytoplasm of the cell.
activation energy (EA)
The energy input necessary to reach the transition state.
activator
A synthesized compound that increases the activity of an enzyme.
active site
The portion of the enzyme that binds substrate and converts it to product.
active transport
The “uphill” movement of substances against a concentration gradient.
adapted
Better able to survive and reproduce in a given environment.
adaptive radiation
A bout of unusually rapid evolutionary diversification in which natural selection accelerates the rates of both speciation and adaptation.
addition rule
The principle that the probability of either of two mutually exclusive outcomes occurring is given by the sum of their individual probabilities.
adenine (A)
A purine base.
adenosine triphosphate (ATP)
The molecule that provides energy in a form that all cells can readily use to perform the work of the cell. ATP is the universal energy currency for all cells.
adherens junction
A beltlike junctional complex composed of cadherins that attaches a band of actin to the plasma membrane.
advantageous
Describes mutations that improve their carriers’ chances of survival or reproduction.
aldose
A monosaccharide with an aldehyde group.
allele frequency
The rate of occurrence of an allele in populations.
alleles
The different forms of a gene, corresponding to different DNA sequences in each different form.
allopatric
Describes populations that are geographically separated from each other.
allosteric effect
A change in the activity or affinity of a protein as the result of binding of a molecule to a site other than the active site.
allosteric enzyme
An enzyme whose activity is affected by binding a molecule at a site other than the active site. Typically, allosteric enzymes change their shape on binding an activator or inhibitor.
alpha (α) carbon
The central carbon atom of each amino acid.
alpha (α) helix
One of the two principal types of secondary structure found in proteins.
alternative splicing
A process in which primary transcripts from the same gene can be spliced in different ways to yield different mRNAs and therefore different protein products.
amino acid
An organic molecule containing a central carbon atom, a carboxyl group, an amino group, a hydrogen atom, and a side chain. Amino acids are the building blocks of proteins.
amino acid replacement
A change in the identity of an amino acid at a particular site in a protein resulting from a mutation in the gene.
amino end
The end of a polypeptide chain that has a free amino group.
amino group
NH2; a nitrogen atom bonded to two hydrogen atoms, covalently linked to the central carbon atom of an amino acid.
aminoacyl (A) site
One of three binding sites for tRNA on the large subunit of a ribosome.
aminoacyl tRNA synthetase
An enzyme that attaches a specific amino acid to a specific tRNA molecule.
amphipathic
Having both hydrophilic and hydrophobic regions.
amplified
In PCR technology, an alternative term for “replicated”.
anabolism
The set of chemical reactions that build molecules from smaller units utilizing an input of energy, usually in the form of ATP. Anabolic reactions result in net energy storage within cells and the organism.
analogous
Describes similar characters that evolved independently in different organisms as a result of adaptation to similar environments.
anaphase
The stage of mitosis in which the sister chromatids separate.
anaphase I
The stage of meiosis I in which the two homologous chromosomes of each bivalent separate as they are pulled in opposite directions, but the sister chromatids remained joined at the centromere.
anaphase II
The stage of meiosis II in which the centromere of each chromosome splits and the separated chromatids are pulled toward opposite poles of the spindle.
anchor
A membrane protein that attaches to other proteins and helps to maintain cell structure and shape.
annealing
The coming together of complementary strands of single-stranded nucleic acids by base pairing.
antiparallel
Oriented in opposite directions; the strands in a DNA duplex are antiparallel.
aquaporin
A protein channel that allows water to flow through the plasma membrane more readily by facilitated diffusion.
Archaea
One of the three domains of life, consisting of single-celled organisms without true chromosomes or a nucleus that divide by binary fission, differing from bacteria in many aspects of their cell and molecular biology.
Ardi
A specimen of Ardipetheciis ramidus, an important early hominin, dating from about 4.4 million years ago.
artificial selection
A form of directional selection analogous to natural selection, but without the competitive element; successful genotypes are determined by the breeder, not by competition.
asexual reproduction
The reproduction of cells or single-celled organisms by cell division; offspring are clones of the parent.
atom
The basic unit of matter.
atomic mass
The mass of the atom determined by the number of protons and neutrons.
ATP synthase
An enzyme that couples the movement of protons through the enzyme with the synthesis of ATP.
autocrine signaling
Signaling between different parts of a cell; the signaling cell and the responding cell are one and the same.
autosome
Any chromosome other than the sex chromosomes.
autotroph
Any organism that is able to convert carbon dioxide into glucose, thus making its own organic source of carbon.
Bacteria
One of the three domains of life, consisting of single-celled organisms without true chromosomes or a nucleus that divide by binary fission, differing from archaeons in many aspects of their cell and molecular biology.
bacteriophage
A virus that infects bacterial cells.
balancing selection
Natural selection that acts to maintain two or more alleles of a given gene in a population.
Baltimore system
A classification of viruses into seven major groups, I-VII, by type of nucleic acid and method of mRNA synthesis; developed by David Baltimore.
band
A crosswise striation in a chromosome, or a horizontal stripe in an electrophoresis gel.
basal lamina
A specialized form of extracellular matrix that underlies and supports all epithelial tissues.
base
A nitrogen-containing compound that makes up part of a nucleotide.
base excision pair
A specialized repair system in which an improper DNA base and its sugar are both removed and the resulting gap is repaired.
base stacking
Stabilizing interactions between bases in the same strand of DNA.
basic
Describes a solution in which the concentration of protons is lower than that of hydroxide ions (the pH is higher than 7).
behaviorally isolated
Describes individuals that only mate with other individuals on the basis of specific courtship rituals, songs, and other behaviors.
beta (β) sheet
One of the two principal types of secondary structure found in proteins.
beta-(β-)oxidation
The process of shortening fatty acids by a series of reactions that sequentially remove two carbon units from their ends.
bilayer
A two-layered structure of the cell membrane with hydrophilic “heads” pointing outward toward the aqueous environment and hydrophobic “tails” oriented inward, away from water.
binary fission
The process by which cells of bacteria or archaeons divide.
binding affinity
The tightness of the binding between the receptor and the signaling molecule.
biological species concept (BSC)
As described by Ernst Mayr, the concept that “[s] pedes are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.” The BSC is the most widely used and accepted definition of a species.
biologist
A scientist who studies life.
biology
The science of life and how it works.
biparental inheritance
A type of inheritance in which the organelles in the offspring cells derive from those in both parents.
bipedal
Moving by two feet and habitually walking upright.
bivalent
The four-stranded structure consisting of two pairs of sister chromatids aligned along their length and held together by chiasmata.
blastocyst
A hollow sphere produced by cells in the morula that move in relation to one another, pushing against and expanding the membrane that encloses them. A blastocyst forms from the blastula, has an inner cell mass, and occurs only in mammals.
blending inheritance
The now-discredited model in which heredity factors transmitted by the parents become intermingled in the offspring instead of retaining their individual genetic identities.
bottleneck
An extreme case of genetic drift that occurs when a population falls to just a few individuals.
Burgess Shale
A sedimentary rock formation on the seafloor covering what is now British Columbia, Canada, that preserves a remarkable sampling of marine life during the initial diversification of animals.
cadherln
A calcium-dependent adherence protein, important in the adhesion of cells to other cells.
Calvin cycle
The process in which carbon dioxide is reduced to synthesize carbohydrates, with ATP and NADPH as the energy sources.
cancer
A group of diseases characterized by uncontrolled cell division.
capsld
The protein coat that surrounds the nucleic acid of a virus.
carbohydrate
An organic molecule containing C, H, and O atoms that provides a source of energy for metabolism and that makes up the cell wall in bacteria, plants, and algae.
carboxyl end
The end of a polypeptide chain that has a free carboxyl group.
carboxyl group
COOH; a carbon atom with a double bond to oxygen and a single bond to a hydroxyl group.
carboxylatlon
The first step of the Calvin cycle, in which carbon dioxide absorbed from the air is added to a 5-carbon molecule.
carrier
A transporter that facilitates movement of molecules.
catabollsm
The set of chemical reactions that break down molecules into smaller units and, in the process, produces ATP to meet the energy needs of the cell.
cell
The simplest self-replicating entity that can exist as an independent unit of life.
cell adhesion molecule
A cell-surface protein that attaches cells to one another and to the extracellular matrix.
cell cycle
The collective name for the steps that make up eukaryotic cell division.
cell division
The process by which cells make more cells.
cell plate
In dividing plant cells, a new cell wall formed in the middle of the cell from the fusing of vesicles during late anaphase and telophase.
cell theory
The theory that the cell is the fundamental unit of life in all organisms and that cells come only from preexisting cells.
cell wall
A defining boundary in many organism, external to the cell membrane, that helps maintain the shape and internal composition of the cell.
cellular blastoderm
In Drosophila development, the structure formed by the nuclei in the single-cell embryo when they migrate to the periphery of the embryo and each nucleus becomes enclosed in its own cell membrane.
cellular junction
A region in the plasma membrane, consisting of cell adhesion molecules and other cytosolic proteins, where a cell makes contact with and adheres to another cell or the extracellular matrix.
cellular respiration
A series of chemical reactions that convert the energy stored in nutrients into a chemical form that can be readily used by cells.
central dogma
The theory that information transfer in a cell usually goes from DNA to RNA to protein.
centromere
A constriction that physically holds sister chromatids together; the site of the attachment of the spindle fibers that move the chromosome in cell division.
centrosome
A compact structure that is the microtubule organizing center for animal cells.
chain terminator
A term for a dideoxynucleotide, which if incorporated into a growing daughter strand stops strand growth because there is no hydroxyl group to attack the incoming nucleotide.
channel
A transporter with a passage that allows the movement of molecules through it.
chaperone
A protein that helps shield a slow-folding protein until it can attain its proper three-dimensional structure.
character
One of the anatomical, physiological, or molecular features that make up organisms’ bodies.
character state
The observed condition of a character, such as presence or absence of lungs or arrangement of petals.
checkpoint
One of multiple regulatory mechanism that coordinate the temporal sequence of events in the cell cycle.
chemical bond
Any form of attraction between atoms that holds them together.
chemical reaction
The process by which molecules are transformed into different molecules.
chemotroph
An organism that derives its energy directly from organic molecules such as glucose.
chiasma
(plural, chiasmata) A crosslike structure within a bivalent constituting a physical manifestation of crossing over.
chlorophyll
The major photosynthetic pigment contained in the thylakoid membrane; it plays a key role in the chloroplast’s ability to capture energy from sunlight. Chlorophyll appears green because it is poor at absorbing green wavelengths.
chloroplast
An organelle that converts energy of sunlight into chemical energy by synthesizing simple sugars.
chloroplast genome
In plant eykaryotic cells, the genome of the chloroplast.
cholesterol
An amphipathic lipid that is a major component of animal cell membranes.
chromatin
A complex of DNA, RNA, and proteins that gives chromosomes their structure; chromatin fibers are either 30 nm in diameter or, in a relaxed state, 10 nm.
chromatin remodeling
The process in which the nucleosomes are repositioned to expose different stretches of DNA to the nuclear environment.
chromosome
In eukaryotes, the physical structure in which DNA in the nucleus is packaged; used more loosely to refer to the DNA in bacterial cells or archaeons.
chromosome condensation
The progressive coiling of the chromatin fiber, an active, energy-consuming process requiring the participation of several types of proteins.
cilium
(plural, cilia) A hairlike organelle that propels the movement of cells or of substances within cells or out of the body; shorter than a flagellum.
cis-regulatory element
A short DNA sequence adjacent to a gene, usually at the 5′ end, that interacts with transcription factors.
cisternae
The series of flattened membrane sacs that make up the Golgi apparatus.
citric acid cycle
The third stage of cellular respiration, in which acetyl-CoA is broken down and more carbon dioxide is released.
cladistics
Phylogenetic reconstruction on the basis of synapomorphies.
class
A group of closely related orders.
clone
An individual that carries an exact copy of the nuclear genome of another individual; clones are genetically identical cells or individuals.
codon
A group of three adjacent nucleotides that specifies an amino acid in a protein or that terminates polypeptide synthesis.
coenzyme Q
The final electron acceptor from both complexes I and II in the electron transport chain.
cofactor
A substance that associates with an enzyme and plays a key role in its function.
combinatorial control
Regulation of gene transcription by means of multiple transcription factors acting together.
comparative genomics
The analysis of the similarities and differences in protein-coding genes and other types of sequence in the genomes of different species.
competitive inhibitor
A reversible inhibitor that reduces the affinity of the substrate for the active site of an enzyme, but can be overcome by excess substrate so the maximum velocity of the reaction is not changed. Competitive inhibitors usually have a structure similar to that of the substrate and therefore bind to the active site of the enzyme.
complementary
Describes the relationship of purine and pyrimidine bases, in which the base A pairs only with T and G pairs only with C.
complex carbohydrate
A long, branched chain of monosaccharides.
complex trait
A trait that is influenced by multiple genes as well as by the environment.
concordance
The percentage of cases in which both members of a pair of twins show the trait when it is known that at least one member shows it.
connective tissue
A type of tissue characterized by a few cells and substantial amounts of extracellular matrix; a major component of the dermis.
conserved
Describes sequences that are similar in different organisms.
constitutive
Describes expression of a gene that occurs continuously.
continuous
Describes variation that occurs across a spectrum.
contractile ring
In animal cells, a ring of actin filaments that forms at the equator of the cell perpendicular to the axis of what was the spindle at the beginning of cytokinesis.
contractile vacuole
A type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment.
copy-number variation (CNV)
Differences among individuals in the number of copies of a region of the genome.
co-speciation
A process in which two groups of organisms speciate in response to each other and at the same time.
covalent bond
A chemical bond formed by a shared pair of electrons holding two different atoms together.
CpG island
A cluster of CpG sites on a DNA strand where cytosine (C) is adjacent to guanosine (G); the “p” represents the phosphate in the backbone.
crisscross inheritance
A pattern in which an X chromosome present in a male in one generation is transmitted to a female in the next generation, and in the generation after that can be transmitted back to a male.
Cro-Magnon
The first population of Homo sapiens to arrive in Europe, named for the site in France where specimens were first described.
crossover
The physical breakage, exchange of parts, and reunion between non-sister chromatids.
C-value paradox
The disconnect between genome size and organismal complexity (the C-value is the amount of DNA in a reproductive cell).
cyclic electron transport
An alternative pathway for electrons during the Calvin cycle that increases the production of ATP.
cyclin-dependent kinase (CDK)
A kinase that is always present within the cell but active only when bound to the appropriate cyclin.
cyclin
A regulatory protein whose levels rise and fall with each round of the cell cycle.
cytochrome b6f complex
Part of the photosynthetic electron transport chain, through which electrons pass between photosystem II and photosystem I.
cytochrome c
The enzyme to which electrons are transferred in complex III of the electron transport chain.
cytokinesis
In eukaryotic cells, the division of the cytoplasm into two separate cells.
cytoplasm
The contents of the cell other than the nucleus.
cytosine
(C) A pyrimidine base.
cytoskeleton
In eukaryotes, an internal protein scaffold that helps cells to maintain their shape and serves as a network of tracks for the movement of substances within cells.
cytosol
The region of the cell inside the plasma membrane but outside the organelles; the jelly-like internal environment that surrounds the organelles.
daughter strand
In DNA replication, the strand synthesized from a parental template strand.
deleterious
Describes mutations that are harmful to an organism.
deletion
A missing region of a gene or chromosome.
denaturation
The unfolding of proteins by chemical treatment or high temperature; the separation of paired, complementary strands of nucleid acid.
deoxyribonucleic acid (DNA)
A linear polymer of four subunits; the information archive in all organisms.
deoxyribose
The sugar in DNA.
dermis
The layer of skin beneath the epidermis, consisting of connective tissue, hair follicles, blood and lymphatic vessels, and glands. It supports the epidermis both physically and by supplying it with nutrients and provides a cushion surrounding the body.
desmosome
A buttonlike point of adhesion that holds the plasma membranes of adjacent cells together.
development
The process in which a fertilized egg undergoes multiple rounds of cell division to become an embryo with specialized tissues and organs.
dideoxynucleotide
A nucleotide lacking both the 2′ and 3′ hydroxyl groups on the sugar ring.
differentiation
The process in which cells become progressively more specialized as a result of gene regulation.
diffusion
The random motion of individual molecules, with net movement occurring where there are areas of higher and lower concentration of the molecules.
dimerization
The mutual attraction of two similar or dissimilar folded polypetide chains that brings them together to form a single molecule.
diploid
Describes a cell with two complete sets of chrmosomes.
directional selection
A form of selection that selects against one of two extremes and leads over time to a change in a trait.
discrete
Describes traits that have clear alternative states.
dispersal
The process in which some individuals colonize a distant place far from the main source population.
disruptive selection
A form of selection that operates in favor of extremes and against intermediate forms, selecting against the mean.
DNA ligase
An enzyme that uses the energy in ATP to close a nick in a DNA strand, joining the 3′ hydroxyl of one end to the 5′ phosphate of the other end.
DNA microarray
A waferlike supporting surface to which are attached millions of different oligonucleotides of known sequence; used in genotyping DNA and measuring levels of gene expression.
DNA polymerase
An enzyme that is a critical component of a large protein complex that carries out DNA replication.
DNA replication
The process of duplicating a DNA molecule, during which the parental strands separate and new partner strands are made.
DNA transposable element (DNA TE)
A sequence that replicates and can move from one location to another in the genome.
DNA typing
The analysis of a small quantity of DNA to uniquely identify an individual.
domain
One of the three largest limbs of the tree of life: Eukarya, Bacteria, or Archaea.
dominant
The trait that appears in the heterozygous offspring of a cross between homozygous genotypes.
donor
In recombinant DNA technology, the source of the DNA fragment that is inserted into a cell of another organism.
dosage
The number of copies of each gene in a chromosome.
dosage compensation
The differential regulation of X- chromosomal genes in females and in males.
double bond
A covalent bond in which covalently joined atoms share two pairs of electrons.
double helix
The structure formed by two strands of complementary nucleotides that coil around each other.
Down syndrome
A condition resulting from the presence of an extra copy of chromosome 21; also known as trisomy 21.
downstream gene
A gene that functions later than another in development.
duplex DNA
Double-stranded DNA.
duplication
A region of a chromosome that is present twice instead of once.
duplication and divergence
The process of creating new genes by duplication followed by progressive change in sequence through evolutionary time.
dynamic instability
Cycles of shrinkage and growth in microtubules.
dynein
A motor protein that carries cargo away from the plasma membrane toward the minus ends of microtubules.
ecological niche
A complete description of the role a species plays in its environment.
ecological separation
The pre-zygotic isolation of both plants and animals in space.
ecological species concept (ESC)
The concept that there is a one-to-one correspondence between a species and its niche.
ecology
The study of how organisms interact with one another and with their physical environment in nature.
ectoderm
The outer germ layer, which differentiates into epithelial cells, pigment cells in the skin, nerve cells in the brain, and the cornea and lens of the eye.
electrochemical gradient
A gradient that combines the charge gradient and the chemical gradient of protons and other ions.
electron
A negatively-charged particle that moves around the atomic nucleus.
electron acceptor
A molecule that gains electrons.
electron carrier
A molecule that stores and transfers energy in the form of “high-energy” or “excited” electrons.
electron donor
A molecule that loses electrons.
electron transport chain
The system that transfers electrons along a series of membrane-associated proteins to a final electron acceptor, releasing the energy of the electrons to produce ATP.
electronegativity
The ability of atoms to attract electrons.
element
A pure substance, such as oxygen, copper, gold, or sodium, that cannot be further broken down by the methods of chemistry.
elongation
The process in protein translation in which successive amino acids are added one by one to the growing polypeptide chain.
elongation factor
A protein that breaks the high-energy bonds of the molecule GTP to provide energy for ribosome movement and elongation of a growing polypeptide chain.
endergonic
Describes reactions with a positive ΔG that are not spontaneous and so require an input of energy.
endocrine signaling
Signaling by molecules that travel through the bloodstream.
endocytosis
The process in which a vesicle buds off from the plasma membrane, bringing material from outside the cell into that vesicle, which can then fuse with other organelles.
endoderm
The germ layer that differentiates into cells of the lining of the digestive tract and lung, liver cells, pancreas cells, and gallbladder cells.
endomembrane system
A cellular system that includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, the plasma membrane, and the vesicles that move between them.
endoplasmic reticulum (ER)
The organelle involved in the synthesis of proteins and lipids.
energetic coupling
The driving of a non-spontaneous reaction by a spontaneous reaction.
enhancer
A specific DNA sequence necessary for transcription.
enthalpy (H)
The total amount of energy in a system.
entropy (S)
The degree of disorder in a system.
envelope
A lipid structure that surrounds the capsids of some viruses.
environmental risk factor
A characteristic in a person’s surroundings that increases the likelihood of developing a particular disease.
environmental variation
Variation among individuals that is due to differences in the environment.
enzyme
A protein that functions as a catalyst to accelerate the rate of a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell.
epidermis
In mammals, the outer layer of skin, which serves as a water-resistant, protective barrier. In plants, sheets of cells that line the leaf’s upper and lower surfaces and constitute the outer cell layer of roots. Also, the outer layer of the cnidarian body.
epigenetic
Describes effects on gene expression due to differences in DNA packaging, such as modifications in histones or chromatin structure.
epistasis
Interaction between genes that modifies the phenotypic expression of genotypes.
epithelial tissue
A type of animal tissue, made up of epithelial cells, that covers the outside of the body and lines many internal structures.
equational division
Another name for meiosis II because cells in meiosis II have the same number of chromosomes at the beginning and at the end of the process.
ethanol fermentation
The fermentation pathway in plants and fungi during which pyruvate releases carbon dioxide to form acetaldehyde and electrons from NADH are transferred to acetaldehyde to produce ethanol and NAD+.
Eukarya
One of the three domains of life, consisting of cells with a true nucleus containing chromosomes that divide by mitosis; cells of Eukarya are eukaryotes.
eukaryote
A cell with a true nucleus containing chromosomes that divide by mitosis.
evolution
Changes in the genetic make-up of populations over time, resulting in some cases in adaptation to the environment and the origin of new species.
evolutionarily conserved
Little changed through evolution and therefore similar from one organism to the next.
evolutionary species concept (EvSC)
The concept that members of a species all share a common ancestry and a common fate.
exergonic
Describes reactions with a negative ΔG that release energy and proceed spontaneously.
exit (E) site
One of three binding sites for tRNA on the large subunit of a ribosome.
exocytosis
The process in which a vesicle fuses with the plasma membrane and empties its contents into the extracellular space or delivers proteins to the plasma membrane.
exon
A sequence that is left intact in mRNA after RNA splicing.
experimentation
A disciplined and controlled way of learning about the world and testing hypotheses in an unbiased manner.
expressed
Turned on or activated, as a gene or protein.
extension
A step in the polymerase chain reaction (PCR) for producing new DNA fragments in which the reaction mixture is heated to the optimal temperature for DNA polymerase, and each primer is elongated by means of deoxynucleoside triphophosphates.
extracellular matrix
A meshwork of proteins and polysaccharides outside the cell; the main constituent of connective tissue.
F1 generation
The first filial, or offspring, generation.
F2 generation
The second filial generation; the offspring of the F1 generation.
facilitated diffusion
Diffusion through a membrane protein, bypassing the lipid bilayer.
family
A group of closely related genera.
fatty acid
A long chain of carbons attached to a carboxyl group; three fatty acid chains attached to glycerol form a triacylglycerol, a lipid used for energy storage.
fermentation
A process of breaking down pyruvate through a wide variety of metabolic pathways that extract energy from fuel molecules such as glucose; the partial oxidation of complex carbon molecules to molecules that are less oxidized than carbon dioxide.
fertilization
The union of gametes to produce a zygote that restores the original chromosomal content of the parental organisms.
first law of thermodynamics
The law of conservation of energy: Energy can neither be created nor destroyed—it can only be transformed from one form into another.
first-division nondisjunction
Failure of chromosome separation in meiosis I.
fitness
A measure of the extent to which an individual’s genotype is represented in the next generation.
fixation
The point at which a given allele has a frequency of 1; it has become the only allele of that gene.
fixed
Describes a population that exhibits only one allele at a particular gene.
flagellum
(plural, flagella) An organelle that propels the movement of cells or of substances within cells; longer than a cilium.
fluid
Describes lipids that are able to move in the plane of the cell membrane.
fluid mosaic model
A model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins.
folding domain
A region of a protein that folds in a similar way across a protein family relatively independently of the rest of the protein.
fossil
The remains of a once-living organism, preserved through time in sedimentary rocks.
founder event
A type of bottleneck that occurs when only a few individuals establish a new population.
frameshift mutation
A mutation in which the insertion or deletion of a single nucleotide causes a one-nucleotide shift in the reading frame of the mRNA, changing all following codons.
fraternal (dizygotic) twins
Twins that arise when two separate eggs, produced by double ovulation, are fertilized by two different sperm.
frequency of recombination
The proportion of recombinant chromosomes among the total number of chromosomes observed.
G protein-coupled receptor
A receptor that couples to G proteins, which bind to the guanine nucleotides GTP and GDP.
G protein
A protein that binds to the guanine nucleotides GTP and GDP.
G0 phase
The gap phase in which cells pause in the cell cycle between M phase and S phase; may last for periods ranging from days to more than a year.
G1 phase
The gap phase in which the size and protein content of the cell increase and specific regulatory proteins are made and activated in preparation for S-phase DNA synthesis.
G2 phase
The gap phase in which the size and protein content of the cell increase in preparation for M-phase mitosis and cytokinesis.
gain-of-function mutation
Any mutation in which a gene is expressed in the wrong place or at the wrong time.
gamete
A reproductive haploid cell resulting from meiotic cell division (in some species gametes are called spores). In many species, there are two types of gametes: eggs in females, sperm in males.
gap junction
A type of connection between the plasma membranes of adjacent animal cells that permits materials to pass directly from the cytoplasm of one cell to the cytoplasm of another.
gastrula
A layered structure formed when the inner cell mass cells of the blastocyst migrate and reorganize.
gastrulation
A highly coordinated set of cell movements in which the cells of the blastoderm migrate inward, creating germ layers of cells within the embryo.
gel electrophoresis
A procedure to determine the size of a DNA fragment, in which DNA samples are inserted into slots or wells in a gel and a current passed through. Fragments move toward the positive pole according to size.
gene
The unit of heredity; the stretch of DNA that affects one or more traits in an organism, usually through an encoded protein or noncoding RNA.
gene expression
The production of a functional gene product.
gene family
A group of genes with related functions, usually resulting from multiple rounds of duplication and divergence.
gene flow
The movement of alleles from one population to another.
gene pool
All the alleles present in all individuals in a species.
gene regulation
The various ways in which cells control gene expression.
general transcription factors
A set of proteins that bind to the promoter of a gene whose combined action is necessary for transcription.
genetic code
The correspondence between codons and amino acids, in which 20 amino acids are specified by 64 codons.
genetic drift
A change in the frequency of an allele due to the random effects of small population size.
genetic incompatibility
Genetic dissimilarity between two organisms, such as different numbers of chromosomes, that is sufficient to act as a pre-zygotic isolating factor.
genetic information
Information carried in DNA, organized in the form of genes.
genetic map
A diagram showing the relative positions of genes along a chromosome.
genetic risk factor
Any mutation that increases the risk of a given disease in an individual.
genetic test
A method of identifying the genotype of an individual.
genetic variation
Differences in genotype among individuals in a population.
genetically modified organism (GMO)
An organism that has been genetically engineered, such as modified viruses and bacteria, laboratory organisms, agricultural crops, and domestic animals. Also known as a transgenic organism.
genome
The genetic material transmitted from a parental cell or organism to its offspring.
genome annotation
The process by which researchers identify the various types of sequence present in genomes.
genotype
The genetic makeup of a cell or organism; the particular combination of alleles present in an individual.
genotype-by-environment interaction
Variation in the effects of the environment on different genotypes, resulting in different phenotypes.
genus
A group of closely related species.
geologic timescale
The series of time divisions that mark Earth’s long history.
germ cells
The reproductive cells that produce sperm or eggs and the cells that give rise to them.
germ layers
Three sheets of cells, the ectoderm, mesoderm, and endoderm, formed by migrating cells of the gastrula, that differentiate further into specialized cells.
germ-line mutation
A mutation that occurs in eggs and sperm or in the cells that give rise to these reproductive cells and therefore is passed on to the next generation.
Gibbs free energy (G)
The amount of energy available to do work.
glycerol
A 3-carbon molecule with OH groups attached to each carbon.
glycogen
The form in which glucose is stored in animals.
glycolysis
The breakdown of glucose to pyruvate; the first stage of cellular respiration. (The second of the four stages is the conversion of pyruvate to acetyl-CoA and the release of CO2.)
glycosidic bond
A covalent bond that attaches one monosaccharide to another.
Golgi apparatus
The organelle that modifies proteins and lipids produced by the ER and acts as a sorting station as they move to their final destinations.
grana
Interlinked structures that form the thylakoid membrane.
growth factor
Any one of a group of small, soluble molecules, usually the signal in paracrine signaling, that affect cell growth, cell division, and changes in gene expression.
guanine (G)
A purine base.
hairpin
(structures) Stems-and-loops formed in self-complementary, single-stranded nucleic acid molecules, stabilized by base pairing in the stem.
half-life
The time it takes for half of the atoms in a given sample of a substance to decay.
haploid
Describes a cell with one complete set of chromosomes.
haplotype
A haploid genotype, as the particular combination of alleles present in any particular region of a chromosome.
Hardy-Weinberg equilibrium
A state in which allele and genotype frequencies do not change overtime, suggesting that evolutionary mechanisms are not acting on the population being studied. It also specifies a mathematical relationship between allele frequencies and genotype frequencies.
helicase
A protein that unwinds the parental double helix at the replication fork.
hemidesmosome
A type of desmosome in which integrins are the prominent cell adhesion molecules.
hemophilia
A trait characterized by excessive bleeding that results from a recessive mutation in a gene encoding a protein necessary for blood clotting.
heterotroph
An organism that obtains its carbon from organic molecules synthesized by other organisms.
heterozygote advantage
A form of balancing selection in which the heterozygote’s fitness is higher than that of either of the homozygotes, resulting in selection that ensures that both alleles remain in the population at intermediate frequencies.
heterozygous
Describes an individual who inherits different types of alleles from the parents, or genotypes in which the two alleles for a given gene are different.
hierarchical
Describes gene regulation during development, in which the genes expressed at each stage in the process control the expression of genes that act later.
high-energy phosphate bond
In nucleic acid polymerization, the bond connecting the innermost phosphate to the next. The cleaved bond provides the energy to drive the reaction that creates the phosphodiester bond attaching the incoming nucleotide to the 3′ end of the growing chain.
highly repetitive DNA
A type of noncoding DNA consting of sequences present in many thousands of copies per genome.
histone
A protein found in all eukaryotes that interacts with DNA to form chromatin.
histone code
The pattern of modifications of the histone tails that affects the chromatin structure and gene transcription.
histone tail
A string of amino acids that protrudes from a histone protein in the nucleosome.
homeobox
A DNA sequence within homeotic genes, which function in development, that specifies the homeodomain.
homeodomain
The DNA-binding domain in homeotic proteins, a sequence of 60 amino acids whose sequences are very similar from one homeotic protein to the next.
homeostasis
The active regulation and maintenance, in animals, organs, or cells, of a stable internal physiological state in the face of a changing external environment.
homeotic gene
A gene that specifies the identity of a body part or segment during embryonic development; also known as a Hoxgene.
hominins
A member of one of the different species in the group leading to humans.
homologous
Describes characters that are similar in different species because of descent from a common ancestor.
homologous chromosomes
Pairs of chromosomes, matching in size and appearance, that carry the same set of genes; one of each pair was received from the mother, the other from the father.
homozygous
Describes an individual who inherits an allele of the same type from each parent, or a genotype in which both alleles for a given gene are the same.
horizontal gene transfer
The transfer of genetic material between organisms that are not parent and offspring.
host cell
A cell in which viral reproduction occurs.
hotspot
A site in the genome that is especially mutable.
housekeeping gene
A gene that is transcribed continually because its product is needed at all times and in all cells.
Hox gene
A gene that species the identity of a body part or segment during embryonic development; also known as a homeotic gene.
hybridization
Interbreeding between two different varieties or species of an organism.
hydrogen bond
A weak bond between a hydrogen atom in one molecule and an electronegative atom in another molecule.
hydrophilic
“Water loving”; describes a class of molecules with which water can undergo hydrogen bonding.
hydrophobic
“Water fearing”; describes a class of molecules poorly able to undergo hydrogen bonding with water.
hydrophobic effect
The exclusion of nonpolar molecules by polar molecules, which drives biological processes such as the formation of cell membranes and the folding of proteins.
hypothesis
A tentative explanation for one or more observations that makes predictions that can be tested by experiments or additional observations.
identical (monozygotic) twins
Twins that arise from a single fertilized egg, which after several rounds of cell division, separates into two distinct, but genetically identical, embryos.
inbred line
A true-breeding, homozygous strain.
incomplete dominance
Describes inheritance in which the phenotype of the heterozygous genotype is intermediate between those of homozygous genotypes.
incomplete penetrance
The phenomenon in which some individuals with a genotype corresponding to a trait do not show the phenotype, either because of environmental effects or because of interactions with other genes.
induced pluripotent cell (iPS cell)
A cell that has been reprogrammed to become pluripotent by activation of certain genes, most of them encoding transcription factors or chromatin proteins.
inducer
A small molecule that elicits gene expression.
inhibitor
A synthesized compound that decreases the activity of an enzyme.
initiation
The stage of translation in which methionine is established as the first amino acid in a new polypeptide chain.
initiation factor
A protein that binds to mRNA to initiate translation.
inner cell mass
A mass of cells in one region of the inner wall of the blastocyst from which the body of the embryo develops.
instantaneous speciation
Speciation that occurs in a single generation.
integral membrane protein
A protein that is permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself.
integrin
A transmembrane protein, present on the surface of virtually every animal cell, that enables cells to adhere to the extracellular matrix.
intermediate filament
A polymer of proteins, which vary according to cell type, that combine to form strong, cable-like filaments that provide animal cells with mechanical strength.
intermembrane space
The space between the inner and outer mitochondrial membranes.
interphase
The time between two successive M phases.
intron
A sequence that is excised from the primary transcript and degraded during RNA splicing.
inversion
The reversal of the normal order of a block of genes.
ion
An electrically charged atom or molecule.
ionic bond
The association of two atoms resulting from the attraction of opposite charges.
irreversible inhibitor
Any one of a class of inhibitors that usually forms covalent bonds with enzymes and irreversibly inactivates them.
island population
A distant, isolated population.
isomers
Molecules that have the same chemical formula but different structures.
isotopes
Atoms of the same element that have different numbers of neutrons.
juxtacrine signaling
Signaling by direct physical contact of one cell with another, with no chemical signal that diffuses or circulates through an external medium.
karyotype
A standard arrangement of chromosomes, showing the number and shapes of the chromosomes representative of a species.
ketose
A monosaccharide with a ketone group.
kinesin
A motor protein, similar in structure to myosin, that transports cargo toward the plus end of microtubules.
kinetic energy
The energy of motion.
kinetochore
The protein complexes on a chromatid where spindle fibers attach.
kingdom
A group of closely related phyla.
Klinefelter syndrome
A sex-chromosomal abnormality in which an individual has 47 chromosomes, including two X chromosomes and one Y chromosome.
lactic acid fermentation
The fermentation pathway in animals and bacteria during which electrons from NADH are transferred to pyruvate to produce lactic acid and NAD+.
lagging strand
A daughter strand that has its 5′ end pointed toward the replication fork, so as the parental double helix unwinds, a new DNA piece is initiated at intervals, and each new piece is elongated at its 3′ end until it reaches the piece in front of it.
lariat
A loop and tail of RNA formed after RNA splicing.
lateral inhibition
Inhibition of a process in cells adjacent to the cell receiving a signal inducing that process, enhancing the strength of a signal locally but diminishing it peripherally.
leading strand
A daughter strand that has its 3′ end pointed toward the replication fork, so as the parental double helix unwinds, this daughter strand can be synthesized as one long, continuous polymer.
ligand
Alternative term for a signaling molecule that binds with a receptor, usually a protein.
ligand-binding site
The specific location on the receptor protein where a signaling molecule binds.
ligand-gated ion channel
A receptor that alters the flow of ions across the plasma membrane when bound by its ligand.
LINE
Long interspersed nuclear element of about 1000 base pairs present in multiple copies in a genome owing to transposition.
linked
Describes genes that are sufficiently close together in the same chromosome that they do not assort independently.
lipid
An organic molecule that stores energy, acts as a signaling molecule, and is a component of cell membranes.
lipid raft
Lipids assembled in a defined patch in the cell membrane.
liposome
An enclosed bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water.
lock and key
In mating, describes a system that requires both components, whether physical or biochemical, to match for a successful interaction to take place.
loss-of-function mutation
A mutation that inactivates the normal function of a gene.
LTR element
A type of transposable element characterized by long repeated sequences, called long terminal repeats, at its ends, and that transposes by means of an RNA intermediate.
Lucy
An unusually complete early hominin fossil, found in 1974 in Ethiopia.
lumen
In eukaryotes, the continuous interior of the endoplasmic reticulum; in plants, a fluid-filled compartment enclosed by the thylakoid membrane; generally, the interior of any tubelike structure.
lysogenic pathway
The alternative to the lytic pathway; in the lysogenic pathway, a virus integrates its DNA into the host cell’s DNA, which is then transmitted to offspring cells.
lysosome
A vesicle derived from the Golgi apparatus that contains enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates.
lytic pathway
The alternative to the lysogenic pathway; in the lytic pathway, a virus bursts, or lyses, the cell it infects, releasing new virus particles.
M phase
The stage of the cell cycle consisting of mitosis and cytokinesis, in which the parent cell divides into two daughter cells.
mainland population
The central population of a species.
major groove
The larger of two uneven grooves on the outside of a DNA duplex.
MAP kinase pathway
A series of kinases that are triggered by activated GTP-bound Ras; the final kinase enters the nucleus, where it phosphorylates its target proteins.
map unit
A unit of distance in a genetic map equal to the distance between genes resulting in 1% recombination.
mass extinction
A catastrophic drop in recorded diversity, which has occurred several times in the past 542 million years.
maternal inheritance
A type of inheritance in which the organelles in the offspring cells derive from those in the mother.
maternal-effect gene
A gene that is expressed by the mother that affects the phenotype of the offspring, typically through the composition or organization of the oocyte.
mediator complex
A complex of proteins that interacts with the Pol II complex and allows transcription to begin.
meiosis I
Reductional division, the first stage of meiotic cell division, in which the number of chromosomes is halved.
meiosis II
Equational division, the second stage of meotic cell division, in which the number of chromosomes is unchanged.
meiotic cell division
A form of cell division that includes only one round of DNA replication but two rounds of nuclear division; meiotic cell division makes sexual reproduction possible.
membrane potential
A difference in electrical charge across the plasma membrane.
mesoderm
The intermediate germ layer, which differentiates into cells that make up connective tissue, muscle cells, red blood cells, bone cells, kidney cells, and gonad cells.
Messel Shale
A sedimentary rock formation in a lake in Germany, preserving fossils that document fish, birds, mammals, and reptiles from the beginning of the age of mammals.
messenger RNA (mRNA)
The RNA molecule that combines with a ribosome to direct protein synthesis; it carries the genetic “message” from the DNA to the ribosome.
metabol ism
The chemical reactions that convert molecules into other molecules and transfer energy in living organisms.
metaphase
The stage of mitosis in which the chromosomes are aligned in the middle of the dividing cell.
metaphase I
The stage of meiosis I in which the meiotic spindle is completed and the bivalents move to lie on an imaginary plane cutting transverely across the spindle.
metaphase II
The stage of meiosis II in which the chromosomes line up so that their centromeres lie on an imaginary plane cutting across the spindle.
micelle
A spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed.
microfilament
A helical polymer of actin monomers, present in various locations in the cytoplasm, that helps make up the cytoskeleton.
microRNA
Small, regulatory RNA molecules that can inhibit translation; also called miRNA.
microtubule
A hollow, tubelike polymer of tubulin dimers that helps make up the cytoskeleton.
migration
The movement of individuals from one population to another.
minor groove
The smaller of two unequal grooves on the outside of a DNA duplex.
mismatch repair
The correction of a mismatched base in a DNA strand by cleaving one of the strand backbones, degrading the sequence with the mismatch, and resynthesizing from the intact DNA strand.
mitochondria
Specialized organelles that harness energy for the cell from chemical compounds like sugars and convert it into ATP.
mitochondrial DNA (mtDNA)
A small circle of DNA, about 17,000 base pairs long, found in every mitochondrion.
mitochondrial genome
In eukaryotic cells, the DNA in the mitochondria.
mitochondrial matrix
The space enclosed by the inner membrane of the mitochondria
mitosis
In eukaryotic cells, the division of the nucleus, in which the chromosomes are separated into two nuclei.
mitotic spindle
A structure in the cytosol made up predominantly of microtubules that pull the chromosomes into separate daughter cells.
moderately repetitive
DNA A type of noncoding DNA consisting of repeated sequences present in hundreds of copies per genome.
Modern Synthesis
The current theory of evolution, which combines Darwin’s theory of natural selection and Mendelian genetics.
molecular clock
The relative constancy of rates of evolutionary change in a DNA nucleotide sequence or a protein amino acid sequence; the correlation between the time two species have been evolutionarily separated and the amount of genetic divergence between them.
molecular evolution
Evolution at the level of DNA, which in time results in the genetic divergence of populations.
molecular fossils
Sterols, bacterial lipids, and some pigment molecules, which are relatively resistant to decomposition, that accumulate in sedimentary rocks and document organisms that rarely form conventional fossils.
molecular orbital
A merged orbital traversed by a pair of shared electrons.
molecular self-assembly
The process by which, when conditions and relative amounts are suitable, viral components spontaneously interact and assemble into mature virus particles.
molecule
A substance made up of two or more atoms.
monophyletic
Describes groupings in which all members share a single common ancestor not shared with any other species or group of species.
monosaccharide
A simple sugar.
morphospecies concept
The principle that members of the same species usually look alike.
morula
The solid ball of cells resulting from early cell divisions of the fertilized egg.
motor protein
A small accessory protein that causes muscle contraction by moving the actin microfilaments inside muscle cells.
multiple alleles
Two or more different alleles of the same gene, occurring in a population of organisms.
multiplication rule
The principle that the probability of two independent events occurring together is the product of their respective probabilities.
multipotent
Describes cells that can form a limited number of types of specialized cell.
multiregional hypothesis
The idea that modern humans derive from the Homo ergaster populations that spread around the world starting 2 million years ago.
mutagen
An agent that increases the probability of mutation.
mutation
Any heritable change in the genetic material, usually a change the nuelcotide sequence of a gene.
myosin
A motor protein found in muscle cells that carries cargo to the plus ends of microfilaments; used by both striated and smooth muscles to contract and generate force.
natural selection
The process in which, when there is inherited variation in a population of organisms, the variants best suited for growth and reproduction in a given environment contribute disproportionately to future generations. Of all the evolutionary mechanisms, natural selection is the only one that leads to adaptations.
Neanderthal
Homo neanderthalensis, a species similar to humans, but with thicker bones and flatter heads that contained brains about the same size as humans′; appeared in the fossil record 60,000-30,000 years ago.
negative feedback
Describes the effect in which the final product of a biochemical pathway inhibits the first step; the process in which a stimulus acts on a sensor that communicates with an effector, producing a response that opposes the initial stimulus. Negative feedback is used to maintain steady conditions, or homeostasis.
negative regulation
The process in which a regulatory molecule must bind to the DNA at a site near the gene to prevent transcription.
negative selection
Natural selection that reduces the frequency of a deleterious allele.
neoteny
The long-term evolutionary process in which the timing of development is altered so that a sexually mature organism retains the physical characteristics of the juvenile form.
neutral
Describes mutations that have no effect or negligible effects on the organism, or whose effects are not associated with differences in survival or reproduction.
neutron
An electrically neutral particle in the atomic nucleus.
nicotinamide adenine dinucleotide phosphate (NADPH)
An important cofactor in many biosynthetic reactions; the reducing agent used in the Calvin cycle.
node
In phylogenetic trees, the point where a branch splits, representing the common ancestor from which the descendant species diverged. In plants, the point on a shoot where one or more leaves are attached.
non-competitve inhibitor
A reversible inhibitor that reduces the maximum velocity of the reaction, but does not affect the affinity of the substrate for the active site of the enzyme. It usually has a structure very different from that of the substrate and binds to the enzyme at a site different from the active site.
nondisjunction
The failure of a pair of chromosomes to separate normally during anaphase of cell division.
non-random mating
Mate selection in which genotype is a factor.
nonrecombinants
Progeny in which the alleles are present in the same combination as that present in a parent.
nonsense mutation
A mutation that creates a stop codon, terminating translation.
non-sister chromatids
Chromatids of different members of a pair of homologous chromosomes; although they carry the same complement of genes, they are not genetically identical.
nonsynonymous (missense) mutation
A point mutation (nucleotide substitution) that causes an amino acid replacement.
nontemplate strand
The untranscribed partner of the template strand of DNA used in transcription.
norm of reaction
A graphical depiction of the change in phenotype across a range of environments.
normal distribution
A distribution whose plot is a bell-shaped curve.
nuclear envelope
The cell structure, composed of two membranes, inner and outer, that defines the boundary of the nucleus.
nuclear genome
In eukaryotic cells, the DNA in the chromosomes.
nuclear localization signal
The signal sequence for the nucleus that enables proteins to move through pores in the nuclear envelope.
nuclear pore
One of many protein channels in the nuclear envelope that act as gateways that allow molecules to move into and out of the nucleus and are thus essential for the nucleus to communicate with the rest of the cell.
nuclear transfer
A procedure in which a hollow glass needle is used to insert the nucleus of a cell into the cytoplasm of an egg whose own nucleus has been destroyed or removed.
nucleic acid
A polymer of nucleotides that encodes and transmits genetic information.
nucleoid
In prokaryotes, a cell structure with multiple loops formed from supercoils of DNA.
nucleoside
A molecule consisting of a 5-carbon sugar and a base.
nucleosome
A beadlike repeating unit of histone proteins wrapped with DNA making up the 10-nm chromatin fiber.
nucleotide
The subunit of nucleic acids, each consisting of a 5-carbon sugar, a nitrogen-containing base, and one or more phosphate groups.
nucleotide excision repair
The repair of multiple mismatched or damaged bases across a region; a process similar to mismatch repair, but over a much longer piece of DNA, sometimes thousands of nucleotides.
nucleotide substitution
A mutation in which a base pair is replaced by a different base pair; this is the most frequent type of mutation. Also known as a point mutation.
nucleus
(of a cell) The compartment of the cell that houses the DNA in chromosomes.
nucleus
(of an atom) The dense central part of an atom containing protons and neutrons.
observation
The act of viewing the world around us.
Okazaki fragment
Any one of the many short DNA pieces in the lagging strand.
oligonucleotide
A short (typically 20 to 3 0 nucleotides), single-stranded molecule of known sequence produced by chemical synthesis; oligonucleotides are often used as primer sequences in the polymerase chain reaction.
oncogene
A cancer-causing gene.
oocyte
The unfertilized egg cell produced by the mother; the developing female gamete.
open reading frame (ORF)
A stretch of DNA or RNA consisting of codons for amino acids uninterrupted by a stop codon. In genome annotation, this sequence motif identifies the region as potentially protein coding.
operator
The binding site for a repressor protein.
operon
A group of functionally related genes located in tandem along the DNA and transcribed as a single unit from one promoter; the region of DNA consisting of the promoter, the operator, and the coding sequence for the structural genes.
orbital
A region in space where an electron is present most of the time.
order
A group of closely related families.
organ
Two or more tissues that combine and function together.
organelle
Any one of several compartments in eukaryotes that divide the cell contents into smaller spaces specialized for different functions.
organic molecule
A carbon-containing molecule.
origin of replication
Each point on a DNA molecule at which DNA synthesis is initiated.
osmosis
The net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration.
Out-of-Africa hypothesis
The idea that modern humans arose from Homo ergaster descendants in Africa about 200,000 years ago.
oxidation reaction
A reaction in which a molecule loses electrons and releases energy.
oxidation-reduction reaction
A reaction involving the loss and gain of electrons between reactants. In biological systems these reactions are often used to store or release chemical energy.
oxidative phosphorylation
The fourth stage of cellular respiration, in which electron carriers generated in stages 1-3 donate their high-energy electrons to an electron transport chain.
oxidizing agent
An electron acceptor.
P1 generation
The parental generation in a series of crosses.
palindromic
Describes sequence identity in the paired strands of a duplex DNA molecule; a symmetry typical of restriction sites.
paracrine signaling
Signaling by a molecule that travels a short distance to the nearest neighboring cell to bind its receptor and deliver its message.
paraphyletic
Describes groupings that include some, but not all, the descendants of a common ancestor.
parsimony
Choosing the simplest hypothesis to account for a given set of observations.
partially reproductively isolated
Describes populations that have not yet diverged as separate species but whose genetic differences are extensive enough that the hybrid offspring they produce have reduced fertility or viability compared to offspring produced by crosses between individuals within each population.
paternal inheritance
A type of inheritance in which the organelles in the offspring cells derive from those in the father.
pedigree
A diagram of family history that summarizes the record of the ancestral relationships among individuals.
peptide bond
A covalent bond that links the carbon atom in the carboxyl group of one amino acid to the nitrogen atom in the amino group of another amino acid.
peptidyl (P) site
One of three binding sites for tRNA on the large subunit of a ribosome.
periodic table of the elements
The arangement of the chemical elements in tabular form, organized by their chemical properties.
peripatric speciation
A specific kind of allopatric speciation in which a few individuals from a mainland population disperse to a new location remote from the original population and evolve separately.
peripheral membrane protein
A protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions.
personalized medicine
An approach in which the treatment is matched to the patient, not the disease; examination of an individual’s genome sequence, by revealing his or her disease susceptibilities and drug sensitivities, allows treatments to be tailored to that individual.
phenotype
The expression of a physical, behavioral, or biochemical trait; an individual’s observable phenotypes include height, weight, eye color, and so forth.
phosphatase
An enzyme that removes a phosphate group from another molecule.
phosphate group
A chemical group consisting of a phosphorus atom bonded to four oxygen atoms.
phosphodiester bond
A bond that forms when a phosphate group in one nucleotide is covalently joined to the sugar unit in another nucleotide. Phosophodiester bonds are relatively stable and form the backbone of a DNA strand.
phospholipid
A type of lipid and a major component of the cell membrane.
photic zone
The surface layer of the ocean through which enough sunlight penetrates to enable photosynthesis.
photorespiration
The process in which ATP is used to drive the reactions that convert a portion of the carbon atoms in 2-phosphoglycolate into 3-phosphoglyceric acid, which can reenter the Calvin cycle.
photosynthesis
The biochemical process in which carbohydrates are built from carbon dioxide and the energy of sunlight; oxygen is released as a waste product.
photosynthetic electron transport chain
A series of redox reactions in which electrons are passed from one compound to another.
photosystem
A protein-pigment complex that absorbs light energy to drive redox reactions and thereby sets the photosynthetic electron transport chain in motion.
photosystem I
The photosystem that energizes electrons with a second input of light energy so they have enough energy to reduce NADP+.
photosystem II
The photosystem that supplies electrons to the beginning of the electron transport chain. When photosystem II loses an electron it can pull electrons from water.
phototroph
An organism that captures energy from sunlight.
phragmoplast
In dividing plant cells, a structure formed by overlapping microtubules that guide vesicles containing cell wall components to the middle of the cell.
phylogenetic tree
A diagrammed hypothesis about the evolutionary history, or phylogeny, of a species.
phylogeny
The history of descent with modification and the accumulation of change over time.
phylum
A group of closely related classes, defined by having one of a number of distinctive body plans.
pilus
(plural, pili) A threadlike, hollow structure through which plasmids are transferred between bacteria.
plasma membrane
The membrane that defines the space of the cell, separating the living material within the cell from the nonliving environment around it.
plasmid
In bacteria, a small circular molecule of DNA carrying a small number of genes that can replicate independently of the bacterial genomic DNA.
plasmodesmata
Connections in plant cells between the plasma membranes of adjacent cells that permit materials to pass directly from the cytoplasm of one cell to the cytoplasm of another.
pleiotropy
The phenomenon in which a single gene has multiple effects on seemingly unrelated traits.
pluripotent
Describes embryonic stem cells (cells of the inner mass), which can give rise to any of the three germ layers and therefore to any cell of the body.
point mutation
A mutation in which a base pair is replaced by a different base pair; this is the most frequent type of mutation. Also known as a nucleotide substitution.
Pol II
The RNA polymerase complex responsible for transcription of protein-coding genes.
polar
(molecule) A molecule that has regions of positive and negative charge.
polar body
A small cell produced by the asymmetric first meiotic division of the primary oocyte.
polar covalent bond
Bonds that do not share electrons equally.
polarity
An asymmetry such that one end of a structure differs from the other.
poly(A) tall
The nucleotides added to the 3′ end of the primary transcript by polyadenylation.
polyadenylation
The addition of a long string of consecutive A-bearing ribonucelotideas to the 3′ end of the primary transcript.
polycistronic mRNA
A single molecule of messenger RNA that is formed by the transcription of a group of functionally related genes located next to one another along the bacterial DNA.
polymer
A complex organic molecule made up of repeated simpler units connected by covalent bonds.
polymerase chain reaction (PCR)
A selective and highly sensitive method for making copies of a piece of DNA, which allows a targeted region of a DNA molecule to be replicated into as many copies as desired.
polymorphism
Any genetic difference among individuals sufficiently common that it is likely to be present in a group of 50 randomly chosen individuals.
polypeptide
A polymer of amino acids connected by peptide bonds.
polyphyletic
Describes groupings that do not include the last common ancestor of all members.
polyploidy
The condition of having more than two complete sets of chromosomes in the genome.
polysaccharide
A polymer of simple sugars. Polysaccharides provide long-term energy storage or structural support.
population
All the individuals of a given species that live and reproduce in a particular place; one of several interbreeding groups of organisms of the same species living in the same geographical area.
positive regulation
The process in which a regulatory molecule must bind to the DNA at a site near the gene in order for transcription to take place.
positive selection
Natural selection that increases the frequency of a favorable allele.
posttranslational modification
The modification, after translation, of proteins in ways that regulate their structure and function.
post-zygotic
Describes factors that cause the failure of the fertilized egg to develop into a fertile individual, thus causing reproductive isolation.
potential energy
Stored energy that is released by a change in an object’s structure or position.
prediction
An informed guess about the outcomeof an experiment or observation based on a hypothesis.
pre-zygotic
Describes factors that that prevent the fertilization of an egg, thus causing reproductive isolation.
primary active transport
Active transport that uses the energy of ATP directly.
primary structure
The sequence of amino acids in a protein.
primary transcript
The initial RNA transcript that comes off the template DNA strand.
primate
A member of an order of mammals that share a number of general features that distinguish them from other mammals, including nails rather than claws, front-facing eyes, and an opposable thumb.
primer
A short stretch of RNA at the beginning of each new DNA strand that serves as a starter for DNA synthesis; an oligonucleotide that serves as a starter in the polymerase chain reaction.
principle of independent assortment
The principle that segregation of one set of alleles of a gene pair is independent of the segregation of another set of alleles of a different gene pair.
principle of segregation
The principle by which half the gametes receive one allele of a gene and half receive the other allele.
probability
Among a very large number of observations, the expected proportion of observations that are of a specified type.
probe
A labeled DNA fragment that can be tracked in a procedure such as a Southern blot.
product
Any one of the transformed molecules that result from a chemical reaction.
prokaryote
A unicellular organism without a nucleus. Often used to refer collectively to archaeons and bacteria.
prometaphase
The mitosis in which the nuclear envelope breaks down and the microtubules of the mitotic spindle attach to chromosomes.
promoter
A regulatory region where RNA polymerase and associated proteins bind to the DNA duplex.
proof reading
The process in which DNA polymerases correct their own errors by excising and replacing a mismatched base.
prophase
The stage of mitosis characterized by the appearance of visible chromosomes.
prophase I
The beginning of meiosis I, marked by the visible manifestation of chromosome condensation.
prophase II
The stage of meiosis II in which the chromosomes in the now-haploid nuclei recondense to their maximum extent.
protein family
A group of proteins that are structurally and functionally related.
protein sorting
The process by which proteins end up where they need to be in the cell to perform their function.
proteins
The key structural and functional molecules that do the work of the cell, providing structural support and catalyzing chemical reactions. The term “protein” is often used as a synonym for “polypeptide.”
proton
A positively charged particle in the atomic nucleus.
proto-oncogene
The normal cellular gene counterpart to an oncogene, which is similar to a viral oncogene but can cause cancer only when mutated.
pseudogene
A gene that is no longer functional.
Punnett square
A worksheet in the form of a checkerboard used to predict the consequences of a random union of gametes.
purine
In nucleic acids, the bases adenine and gunanine, which have a double-ring structure.
pyrimidine
In nucleic acids, the bases thymine, cytosine, and uracil, which have a single-ring structure.
quantitative trait
A complex trait in which the phenotype is measured along a continuum with only small intervals between similar individuals.
quaternary structure
The structure that results from the interactions of several polypeptide chains.
R group
A chemical group attached to the central carbon atom of an amino acid, whose structure and composition determine the identity of the amino acid; also known as a residue or side chain.
Ras
A cytoplasmic signaling protein, very similar to the a subunit of G proteins.
reactant
Any of the starting molecules in a chemical reaction.
reaction center
Two specially configured chlorophyll molecules where light energy is converted into electron transport.
reactive oxygen species
Highly reactive forms of oxygen produced when NADP+ is in short supply.
reading frame
Following a start codon, a consecutive sequence of codons for amino acids.
receptor
A molecule on cell membranes that detects critical features of the environment. Receptors detecting signals that easily cross the cell membrane are sometimes found in the cytoplasm.
receptor kinase
A receptor that is an enzyme that adds a phosphate group to another molecule.
receptor molecule
The molecule on the responding cell that binds to the signaling molecule.
recessive
The trait that fails to appear in heterozygous genotypes from a cross between the corresponding homozygous genotypes.
reciprocal cross
A cross in which the female and male parents are interchanged.
reciprocal translocation
Interchange of parts between nonhomologous chromosomes.
recombinant
An offspring with a different combination of alleles from that of either parent, resulting from one or more crossovers in prophase I of meiosis.
recombinant DNA
The joining of DNA molecules from two (or more) different sources into a single molecule.
reducing agent
An electron donor.
reduction
Gain of electrons by a molecule in a reaction; in the second step of the Calvin cycle, energy and electrons are transferred to the molecules formed from carboxylation.
reduction reaction
A reaction in which a molecule acquires electrons and gains energy.
reductional division
An alternative name for meiosis I, since this division reduces the number of chromosomes by half.
redundant
Describes the genetic code, in which many amino acids are specified by more than one codon.
regeneration
The third step of the Calvin cycle, in which the 5-carbon molecule needed for carboxylation is produced.
regenerative medicine
A discipline that aims to use the natural processes of cell growth and development to replace diseased or damaged tissues.
regression toward the mean
With regard to complex traits, the principle that offspring exhibit an average phenotype that is intermediate between that of the parents and that of the population as a whole.
regulatory transcription factor
A protein that recruits the components of the transcription complex to the gene.
reinforcement of reproductive isolation (reinforcement)
The process by which diverging populations undergo natural selection in favor of enhanced pre-zygotic isolation to prevent the production of inferior hybrid offspring.
release factor
A protein that causes the bond connecting the polypeptide to the tRNA to break.
renaturation
The base pairing of complementary single-stranded nucleic acids to form a duplex; also known as hybridization, it is the opposite of denaturation.
replication
The exact copying of DNA so genetic information can be passed from cell to cell or from an organism to its progeny.
replication bubble
A region formed by the opening of a DNA duplex at an origin of replication, which has a replication fork at each end.
replication fork
The site where the parental DNA strands separate as the DNA duplex unwinds.
repressor
A protein that, when bound with the RNA polymerase complex, can turn off transcription.
residue
An amino acid that is incorporated into a protein.
responding cell
The cell that receives information from the signaling molecule.
response
A change in cellular behavior, such as activation of enzymes or genes, following a signal.
restriction enzyme
Any one of a class of enzymes that recognizes specific, short nucleotide sequences in double-stranded DNA and cleaves DNA at or near these sites.
restriction fragment length polymorphism (RFLP)
A polymorphism in which the length of the restriction fragments is different in the two alleles.
restriction site
A recognition sequence in DNA cutting, which is typically four or six base pairs long. Most restriction enzymes cleave double-stranded DNA at or near these restriction sites.
reversible inhibitor
Any molecule in the class of inhibitors that form weak bonds with enzymes and easily dissociate from them.
ribonucleic acid (RNA)
A molecule closely related to DNA that is synthesized by proteins from a DNA template.
ribosomal RNA (rRNA)
Noncoding RNA found in all ribosomes that aid in translation.
ribosome
A complex structure of RNA and protein, bound to the cytosolic face of the RER in the cytoplasm, on which proteins are synthesized.
ribulose bisphosphate carboxylase oxygenase (rubisco)
The enzyme that catalyzes a carboxylation reaction.
ribulose-1,5-bisphosphate (RuBP)
The 5-carbon sugarto which carbon dioxide is added in carboxylation.
ring species
Species that contain populations that are reproductively isolated from each other but can exchange genetic material through other, linking populations.
RISC (RNA-induced silencing complex)
A protein complex that is targeted to specific mRNA molecules by base pairing with short regions on the target mRNA, inhibiting translation or degrading the RNA.
RNA editing
The process in which some RNA molecules become a substrate for enzymes that modify particular bases in the RNA, thereby changing its sequence and what it codes for.
RNA polymerase
The enzyme that carries out polymerization of ribonucleoside triphosphates from a DNA template to produce an RNA transcript.
RNA polymerase complex
Aggregate of proteins that synthesize the RNA transcript complementary to the template strand of DNA.
RNA primase
An RNA polymerase that synthesizes a short piece of RNA complementary to the DNA template and does not require a primer.
RNA processing
Chemical modification that converts the primary transcript into finished mRNA, enabling the RNA molecule to be transported to the cytoplasm and recognized by the translational machinery.
RNA splicing
The process of intron removal from the primary transcript.
RNA transcript
The RNA sequence synthesized from a DNA template.
RNA world hypothesis
The belief that RNA, not DNA, was the original information-storage molecule in the earliest forms of life on Earth.
rough endoplasmic reticulum (RER)
The part of the endoplasmic reticulum with attached ribosomes.
S phase
The phase of interphase in which the entire DNA content of the nucleus is replicated.
saccharide
The simplest carbohydrate molecule, also called a sugar; saccharides store energy in their bonds.
Sanger sequencing
A procedure in which the terminated daughter strands help in determining the DNA sequence.
saturated
Describes fatty acids that do not contain double bonds; the maximum number of hydrogen atoms is attached to each carbon atom, “saturating” the carbons with hydrogen atoms.
scaffold
A supporting protein structure in a metaphase chromosome.
scientific method
A deliberate, careful, and unbiased way of learning about the natural world.
second law of thermodynamics
The principle that the transformation of energy is associated with an increase in the degree of disorder in the universe.
second messenger
An intermediate cytosolic signaling molecule that transmits signals from a receptor to a target within the cell. (First messengers transmit signals from outside the cell to a receptor.)
secondary active transport
Active transport that uses the energy of an electrochemical gradient to drive the movement of molecules.
secondary structure
The structure formed by interactions between stretches of amino acids in a protein.
second-division nondisjunction
Disjunction in the second meiotic division.
segmentation
The formation of discrete parts or segments in the insect embryo.
segregate
Separate; applies to chromosomes or members of a gene pair moving into different gametes.
selection
The retention or elimination of random mutations in a population of organisms.
selective barrier
Describes the plasma membrane, which lets some molecules in and out freely, lets others in and out only under certain conditions, and prevents other molecules from passing through at all.
semiconservative replication
The mechanism of DNA replication in which each strand of a parental DNA duplex serves as a template for the synthesis of a new daughter strand.
sequence assembly
The process in which short nucleotide sequences of a long DNA molecule are arranged in the correct order to generate the complete sequence.
sequence motif
Any of a number of sequences or sequence arrangements that indicate the likely function of a segment of DNA.
sex chromosome
One of the chromosomes associated with sex, in most animals denoted the X and Y chromosomes.
sexual reproduction
The process of producing offspring that receive genetic material from two parents; in eukaryotes, the process occurs through meiosis and fertilization.
sexual selection
A form of selection that promotes traits that increase an individual’s access to reproductive opportunities.
shell
(of an atom) An energy level.
shotgun sequencing
DNA sequencing method in which the sequenced fragments do not originate from a particular gene or region but from sites scattered randomly across the molecule.
sickle-cell anemia
A condition in which hemoglobin molecules tend to crystallize when exposed to lower-than-normal levels of oxygen, causing the red blood cells to collapse and block capillary blood vessels.
side chain
A chemical group attached to the central carbon atom of an amino acid, whose structure and composition determine the identity of the amino acid; also known as an R group.
sigma factor
A protein that associates with RNA polymerase that facilitates its binding to specific promoters.
signal sequence
An amino acid sequence that directs a protein to its proper cellular compartment.
signal transduction
The process in which an extracellular molecule acts as a signal to activate a receptor, which transmits information through the cytoplasm.
signaling cell
The source of the signaling molecule.
signaling molecule
The carrier of information transmitted when the signaling molecule binds to a receptor; also referred to as a ligand.
signal-recognition particle (SRP)
An RNA-protein complex that binds with part of a polypeptide chain and marks the molecule for incorporation into the endoplasmic reticulum (eukaryotes) or the plasma membrane (prokaryotes).
SINE
Any one of many short interspersed nuclear elements of about 300 base pairs present in multiple copies in a genome owing to transposition.
single-gene trait
A trait determined by Mendelian alleles of a single gene without much influence from the environment.
single-nucleotide polymorphism (SNP)
A site in the genome where a base pair differs among individuals in a population.
single-stranded binding protein
A protein that binds single-stranded nucleic acids.
sister chromatids
The two identical copies of chromosomes produced by DNA replication.
sister groups
Groups that are more closely related to each other than either of them is to any other group.
small interfering RNA (si RNA)
A type of small double-stranded regulatory RNA that becomes part of a complex able to cleave and destroy single-stranded RNA with a complementary sequence.
small nuclear RNA (snRNA)
Noncoding RNA found in eukaryotes and involved in splicing, polyadenylation, and other processes in the nucleus.
small regulatory RNA
A short RNA molecule that works primarily by blocking transcription or translation.
smooth endoplasmic reticulum (SER)
The portion of the endoplasmic reticulum that lacks ribosomes.
solvent
A liquid capable of dissolving a substance.
somatic cell
A nonreproductive cell, the most common type of cell in body.
somatic mutation
A mutation that occurs in somatic cells.
Southern blot
A method for determining the size and number of copies of a DNA sequence of interest by means of a labeled probe.
speciation
The process that produces new and distinct forms of life.
species
A group of individuals that can exchange genetic material through interbreeding to produce fertile offspring.
spindle apparatus
The organelle formed by microtubules that separates replicated chromosomes during eukaryotic cell division.
spliceosome
A complex of RNA and protein that catalyzes RNA splicing.
spontaneous
Occurring in the absence of any assignable cause; most mutations are spontaneous.
stabilizing selection
A form of selection that maintains the status quo and selects against extremes.
starch
The form in which glucose is stored in plants.
stem cell
An undifferentiated cell that can undergo an unlimited number of mitotic divisions and differentiate into any of a large number of specialized cell types.
stroma
The region surrounding the thylakoid, where carbohydrate synthesis takes place.
structural gene
A gene that codes for the sequence of amino acids in a polypeptide chain.
subspecies
Allopatric populations that have yet to evolve even partial reproductive isolation but which have accumulated a few population-specific traits.
substrate (S)
A molecule acted upon by an enzyme.
substrate-level phosphorylation
A way of generating ATP in which a phosphate group is transferred to ADP from an organic molecule, which acts as a phosphate donor or substrate.
sugar
The simplest carbohydrate molecule; also called a saccharide.
supercoil
A coil of coils; a circular molecule of DNA can coil upon itself to form a supercoil.
sympatric
Describes populations that are in the same geographic location.
synapomorphy
A shared derived character; a homology shared by some, but not all, members of a group.
synapsis
The gene-for-gene pairing of homologous chromosomes in prophase I of meiosis.
synonymous (silent) mutation
A mutation in a codon that does not alter the corresponding amino acid in the polypeptide.
TATA box
A DNA sequence present in many promoters in eukaryotes and archaeons that serves as a protein-binding site for a key general transcription factor.
taxon
(plural, taxa) All the species in a taxonomic entity such as family or genus.
telomerase
An enzyme that synthesizes telomere repeats.
telomere
A repeating sequence at each end of a eukaryotic chromosome.
telophase
The stage of mitosis in which the nuclei of the daughter cells are formed and the chromosomes uncoil to their original state.
telophase I
The stage of meiosis I in which the chromosomes uncoil slightly, a nuclear envelope briefly reappears, and in many species the cytoplasm divides, producing two separate cells.
telophase II
The stage of meiosis II in which the chromosomes uncoil and become diffuse, a nuclear envelope forms around each set of chromosomes, and the cytoplasm divides by cytokinesis.
template
A strand of DNA or RNA whose squence of nucleotides is used to sythesis a compementary strand.
template strand
In DNA replication, the parental strand whose sequence is used to synthesize a complementary daughter strand.
temporal separation
Separation in time, as the pre-zygotic isolation of both plants and animals in time.
termination
In protein translation, the stage in which the addition of amino acids stops and the completed polypeptide chain is released from the ribosome. In cell communication, the stopping of a signal.
terminator
A DNA sequence at which transcription stops and the transcript is released.
tertiary structure
The overall three-dimensional shape of a protein, formed by interactions between secondary structures.
test
(of a hypothesis) An experiment or observation to determine whether a prediction made by the hypothesis holds true.
testcross
Any cross of an unknown genotype with a homozygous recessive genotype.
tetraplold
A cell or organism with four complete sets of chromosomes; a double diploid.
theory
A general explanation of a natural phenomenon supported by a large body of experiments and observations.
thylakold
The internal membrane-bound compartment in the center of chloroplasts, consisting of the highly folded thylakoid membrane, which contains light-collecting pigments and is the site of the photosynthetic electron transport chain and the interior lumen.
thylakold membrane
A highly folded membrane in the center of the chloroplast that contains light-collecting pigments and that is the site of the photosynthetic electron transport chain.
thymine (T)
A pyrimidine base.
tight junction
A junctional complex that establishes a seal between cells so that the only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by a cellular transport mechanism.
tissue
A collection of cells that work together to perform a specific function.
topoisomerase
Any one of a class of enzymes that regulates the supercoiling of DNA by cleaving one or both strands of the DNA double helix, and later repairing the break.
topoisomerase II
An enzyme that breaks a DNA double helix, rotates the ends, and seals the break.
totipotent
Describes cells that have the potential to give rise to a complete organism; a fertilized egg is a totipotent cell.
trace fossil
A track or trail, such as a dinosaur track or the feeding trails of snails and trilobites, left by an animal as it moves about or burrows into sediments.
trait
A characteristic of an individual.
transcription
The synthesis of RNA from a DNA template.
transcriptional activator protein
A protein that binds to an enhancer to enable transcription to begin.
transcriptional regulation
The mechanisms that collectively regulate whether or not transcription occurs.
transfer RNA (tRNA)
Noncoding RNA that carries individual amino acids for use in translation.
transformation
The conversion of cells from one state to another, as from nonvirulent to virulent, when DNA released to the environment by cell breakdown is taken up by recipient cells. In recombinant DNA technology, the introduction of recombinant DNA into a recipient cell.
transgenic organisms
An alternative term for genetically modified organisms.
transition state
The brief time in a chemical reaction in which chemical bonds in the reactants are broken and new bonds in the product are formed.
translation
Synthesis of a polypeptide chain corresponding to the coding sequence present in a molecule of messanger RNA.
transmembrane proteins
Proteins that span the entire lipid bilayer; most integral membrane proteins are transmembrane proteins.
transmission genetics
The discipline that deals with the manner in which genetic differences among individuals are passed from generation to generation.
transporters
Membrane proteins that move ions or other molecules across the cell membrane.
transposable element
A DNA sequence that can replicate and move from one location to another in a DNA molecule.
transposition
The movement of a transposable element.
tree of life
The full set of evolutionary relationships among all organisms.
triacylglycerol
A lipid that stores energy.
triose phosphate
A 3-carbon carbohydrate molecule; triose phosphates are the true products of the Calvin cycle because they are the molecules exported from the chloroplast.
triploid
A cell or organism with three complete sets of chromosomes.
trisomy 21
A condition resulting from the presence of three, rather than two, copies of chromosome 21; also known as Down syndrome.
true breeding
Describes a trait whose physical appearance in each successive generation is identical to that in the previous one.
tubulin
Dimers (composed of an a tubulin and a b tubulin) that assemble into microfilaments.
tumor suppressors
A family of genes that encode proteins whose normal activities inhibit cell division.
turgor pressure
The pressure exerted by water against an object, which provides structural support for many plants, fungi, and bacteria.
Turner syndrome
A sex-chromosomal abnormality in which an individual has 45 chromosomes, including only one X chromosome.
unbalanced translocation
Tranlocation in which only part of a reciprocal translocation (and one of the nontranslocated chromosomes) is inherited from one of the parents.
unsaturated
Describes fatty acids that contain carbon-carbon double bonds.
uraci I (U)
A pyrimidine base in RNA, where it replaces the thymine found in DNA.
vacuole
A cell structure that absorbs water and contributes to turgor pressure.
valence electrons
The electrons farthest from the nucleus, which are at the highest energy level.
van der Waals forces
The binding of temporarily polarized molecules because of the attraction of opposite charges.
variable
(in experimentation) The feature of an experiment that is changed by the experimenter from one treatment to the next.
variable expressivity
The phenomenon in which a particular phenotype is expressed with a different degree of severity in different individuals.
variable number tandem repeat (VNTR)
A genetic difference in which the number of short repeated sequences of DNA differs from one chromosome to the next.
vector
In recombinant DNA, a carrier of the donor fragment, usually a plasmid.
vesicle
A small membrane-enclosed sac that transports substances within the cell.
vicariance
The process in which a geographic barrier arises within a single population, separating it into two or more isolated populations.
virus
A small infectious agent that contains a nucleic acid genome packaged inside a protein coat called a capsid.
visible light
The portion of the electromagnetic spectrum apparent to our eyes.
wild type
The most common allele, genotype, or phenotype present in a population; nonmutant.
X chromosome
One of the two sex chromosomes; a normal human female has two copies of the X chromosome, a normal male has one X and one Y chromosome.
xanthophyll
Any one of several yellow-orange pigments that slow the formation of reactive oxygen species by reducing excess light energy; these pigments accept absorbed light energy directly from chlorophyll and convert this energy to heat.
X-inactivation
The process in mammals in which dosage compensation occurs through the inactivation of one X chromosome in each cell in females.
X-linked gene
A gene in the X chromosome.
Y chromosome
One of the two sex chromosomes; a normal human male has one X and one Y chromosome.
Y-linked gene
A gene that is present in the region of the Y chromosome that shares no homology with the X chromosome.
Z scheme
Another name for the photosynthetic electron transport chain, so called because the overall energy trajectory resembles a “Z.”
zygote
The diploid fertilized egg cell formed by the fusion of two haploid gametes.