Glossary - R

Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

radioisotope Unstable form of an atom that emits radiation as it decays. Several radioisotopes are commonly used experimentally as labels in biological molecules. (Table 3-1)

Ras protein A monomeric member of the GTPase superfamily of switch proteins that is tethered to the plasma membrane by a lipid anchor and functions in intracellular signaling pathways; activated by ligand binding to receptor tyrosine kinases and some other cell-surface receptors. (Figures 16-21 and 16-23)

rate constant A constant that relates the concentrations of reactants to the rate of a chemical reaction.

Rb protein Inhibitor of the E2F transcription factor family and thus a key regulator of cell cycle entry.

reading frame The sequence of nucleotide triplets (codons) that runs from a specific translation start codon in an mRNA to a stop codon. Some mRNAs can be translated into different polypeptides by reading in two different reading frames. (Figure 5-18)

receptor Any protein that specifically binds another molecule to mediate cell-cell signaling, adhesion, endocytosis, or other cellular process. Commonly denotes a protein located in the plasma membrane, cytosol, or nucleus that binds a specific extracellular molecule (ligand), which often induces a conformational change in the receptor, thereby initiating a cellular response. See also adhesion receptor and nuclear receptor. (Figures 15-1 and 16-1)

receptor-mediated endocytosis Uptake of extracellular materials bound to specific cell-surface receptors by invagination of the plasma membrane to form a small membrane-bounded vesicle (early endosome). (Figure 14-29)

receptor tyrosine kinase (RTK) Member of a large class of cell-surface receptors, usually with a single transmembrane domain, including those for insulin and many growth factors. Ligand binding activates tyrosine-specific protein kinase activity in the receptor’s cytosolic domain, thereby initiating intracellular signaling pathways. (Figures 16-14 and 16-15)

recessive In genetics, referring to that allele of a gene that is not expressed in the phenotype when the dominant allele is present; also refers to the phenotype of an individual (homozygote) carrying two recessive alleles. Mutations that produce recessive alleles generally result in a loss of the gene’s function. (Figure 6-2)

recombinant DNA Any DNA molecule formed in vitro by joining DNA fragments from different sources.

recombination Any process in which chromosomes or DNA molecules are cleaved and the fragments are rejoined to give new combinations. Homologous recombination occurs during meiosis, giving rise to crossing over of homologous chromosomes. Homologous recombination and nonhomologous recombination (i.e., between chromosomes of different morphologic type) also occur during several DNA-repair mechanisms and can be carried out in vitro with purified DNA and enzymes. (Figure 6-10)

redox reaction An oxidation-reduction reaction in which one or more electrons are transferred from one reactant to another.

reduction Gain of electrons by an atom or molecule as occurs when a hydrogen atom is added to a molecule or oxygen is removed. The opposite of oxidation.

reduction potential (E) The voltage change when an atom or molecule gains an electron; a measure of the tendency of a molecule to gain an electron. For a given reduction reaction, E has the same magnitude but opposite sign as the oxidation potential for the reverse (oxidation) reaction.

release factor (RF) One of two types of nonribosomal proteins that recognize stop codons in mRNA and promote release of the completed polypeptide chain, thereby terminating translation (protein synthesis). (Figure 5-26)

replication forkY-shaped region in double-stranded DNA at which the two strands are separated and replicated during DNA synthesis; also called growing fork. (Figure 5-29)

replication origin Unique DNA segment present in an organism’s genome at which DNA replication begins. Eukaryotic chromosomes contain multiple origins, whereas bacterial chromosomes and plasmids usually contain just one.

reporter gene A gene encoding a protein that is easily assayed (e.g., β-galactosidase, luciferase). Reporter genes are used in various types of experiments to indicate activation of the promoter to which the reporter gene is linked.

repression domain A region of a repressor transcription factor that will inhibit transcription when fused to a DNA-binding domain.

repressor Specific transcription factor that inhibits transcription.

residue General term for the repeating units in a polymer that remain after covalent linkage of the monomeric precursors.

resolution The minimum distance between two objects that can be distinguished by an optical apparatus; also called resolving power.

respiration The conversion of energy in nutrients into ATP via a set of reactions involving a series of protein-catalyzed, membrane-associated oxidation and reduction reactions, called an electron transport chain, that are ultimately coupled to the addition of Pi to ADP (oxidative phosphorylation) to form ATP and transfer of electrons to oxygen or other inorganic electron acceptors.

G-23

respiratory chain See electron transport chain.

respiratory control Dependence of mitochondrial oxidation of NADH and FADH2 on the supply of ADP and Pi for ATP synthesis.

resting K+ channels Nongated K+ ion channels in the plasma membrane that, in conjunction with the high cytosolic K+ concentration produced by the Na+/K+ ATPase, are primarily responsible for generating the inside-negative resting membrane potential in animal cells.

restriction enzyme Any enzyme that recognizes and cleaves a specific short sequence, the restriction site, in double-stranded DNA molecules; used extensively to produce recombinant DNA in vitro; also called restriction endonuclease. (Figure 6-11; Table 6-1)

restriction point Point in the mammalian cell cycle after which cells are no longer responsive to proliferation regulatory signals.

retrotransposon Type of eukaryotic transposable DNA element whose movement in the genome is mediated by an RNA intermediate and involves a reverse-transcription step. See also transposon. (Figure 8-8b)

retrovirus Type of eukaryotic virus containing an RNA genome that replicates in cells by first making a DNA copy of the RNA. This viral DNA is inserted into cellular chromosomal DNA, forming a provirus, and gives rise to further genomic RNA as well as the mRNAs for viral proteins. (Figure 5-48)

reverse transcriptase Enzyme found in retroviruses that catalyzes a complex reaction in which a double-stranded DNA is synthesized from a single-stranded RNA template. (Figure 8-14)

R group A portion of a molecule, or a chemical group within a larger molecule, that is covalently bonded as an appendage to the main body or core of the molecule. In an amino acid, the R group is the side chain that is attached to the alpha carbon atom and that confers the distinct characteristics of the amino acid.

ribonucleic acid (RNA) See RNA.

ribonucleoprotein (RNP) complex General term for any complex composed of proteins and RNA. Most RNA molecules are present in the cell in the form of RNPs.

ribosome A large complex comprising several different rRNA molecules and as many as 83 proteins, organized into a large subunit and small subunit; the engine of translation (protein synthesis). (Figures 5-22 and 5-23)

ribosomal RNA See rRNA.

ribozyme An RNA molecule with catalytic activity. Ribozymes function in RNA splicing and protein synthesis.

ribulose 1,5-bisphosphate carboxylase Enzyme located in chloroplasts that catalyzes the first reaction in the Calvin cycle, the addition of CO2 to a five-carbon sugar (ribulose 1,5-bisphosphate) to form two molecules of 3-phosphoglycerate; also called rubisco. (Figure 12-47)

RISC See RNA-induced silencing complex.

RNA (ribonucleic acid) Linear, single-stranded polymer, composed of ribose nucleotides. mRNA, rRNA, and tRNA play different roles in protein synthesis; a variety of small RNAs play roles in controlling the stability and translation of mRNAs and in controlling chromatin structure and transcription. (Figure 5-17)

RNA editing Unusual type of RNA processing in which the sequence of a pre-mRNA is altered.

RNA-induced silencing complex (RISC) Large multiprotein complex associated with a short single-stranded RNA (siRNA or miRNA) that mediates degradation or translational repression of a complementary or near-complementary mRNA.

RNA interference (RNAi) Functional inactivation of a specific gene by a corresponding double-stranded RNA that induces either inhibition of translation or degradation of the complementary single-stranded mRNA encoded by the gene but not that of mRNAs with a different sequence. (Figure 6-42)

RNA polymerase An enzyme that copies one strand of DNA (the template strand) to make the complementary RNA strand using as substrates ribonucleoside triphosphates. (Figure 5-11)

RNA splicing A process that results in removal of introns and joining of exons in pre-mRNAs. See also spliceosome. (Figures 10-8 and 10-9)

rRNA (ribosomal RNA) Any one of several large RNA molecules that are structural and functional components of ribosomes. Often designated by their sedimentation coefficient: 28S, 18S, 5.8S, and 5S rRNA in higher eukaryotes. (Figure 5-22)

rubisco See ribulose 1,5-bisphosphate carboxylase.