Rab protein Any of a large family of monomeric GTPases present in the
plasma membrane and organelle membranes that are involved in conferring
specificity on vesicle docking.
radioactive isotope Form of an atom with an unstable nucleus that emits
radiation as it decays.
Ran Monomeric GTPase present in both cytosol and nucleus that is required
for the active transport of macromolecules into and out of the nucleus through
nculear pore complexes. Hydrolysis of GTP to GDP is thought to provide the
energy required for this transport.
Ras protein The most famous member of a large family of GTP-binding
proteins (called monomeric GTPases) that help relay signals from cell-surface
receptors to the nucleus. Named for the ras gene, first identified in viruses that
cause rat sarcomas.
reaction In chemistry, any process in which one molecule is converted into
another by the removal or addition of atoms, or in which the arrangement of
atoms in a molecule or molecules is altered by a change in chemical bonds.
reading frame The phase in which nucleotides are read in sets of three to
encode a protein. A messenger RNA molecule can be read in any one of three
reading frames, only one of which will give the required protein.
RecA protein The prototype for a class of DNA-binding proteins that catalyze
synapsis of DNA strands during genetic recombination.
receptor Protein that binds a specific extracellular signal molecule (ligand)
and initiates a response in the cell. Cell-surface receptors, such as the
acetylcholine receptor and the insulin receptor, are located in the plasma
membrane, with their ligand-binding site exposed to the external medium.
Intracellular receptors, such as steroid hormone receptors, bind ligands that
diffuse into the cell across the plasma membrane.
receptor-mediated endocytosis Internalization of receptor-ligand complexes
from the plasma membrane by endocytosis, It is used to take up some
macromolecules, such as cholesterol-containing lipoproteins, from the
extracellular fluid, and is also a means of recycling receptor proteins once they
have bound their ligands.
recessive In genetics, refers to the member of a pair of alleles that fails to be
expressed in the phenotype of the organism when the dominant allele is
present. Also refers to the phenotype of an individual that has only the
recessive allele.
recombinant DNA Any DNA molecule formed by joining DNA segments
from different sources. Recombinant DNAs are widely used in the cloning of
genes, in the genetic modification of organisms, and in molecular biology
generally.
recombination Process in which DNA molecules are broken and the
fragments are rejoined in new combinations. Can occur in the living cell for
example, through crossing-over during meiosis or in vitro using purified
DNA and enzymes that break and ligate DNA strands.
recycling endosomes Large intracellular membrane-bounded vesicle formed
from a fragment of an endosome that is an intermediate stage on the passage of
recycled receptors back to the cell membrane.
red blood cell see erythrocyte
redox pair Pair of molecules in which one acts as an electron donor and one as
an electron acceptor in an oxidation-reduction reaction; for example, NADH
(electron donor) and NAD+ (electron acceptor).
redox potential The affinity of a redox pair for electrons, generally measured
as the voltage difference between an equimolar mixture of the pair and a
standard reference. NADH/NAD+ has a low redox potential and O2/H2 has a
high redox potential (high affinity for electrons).
redox reaction A reaction in which one component becomes oxidized and the
other reduced; an oxidation-reduction reaction.
reduction (verb reduce) Addition of electrons to an atom, as occurs during
the addition of hydrogen to a molecule or the removal of oxygen from it.
Opposite of oxidation. (2 43.)
regulatory sequence DNA sequence to which a gene regulatory protein binds
to control the rate of assembly of the transcirptional complex at the promoter.
regulatory site Site on an enzyme, other than the active site, that binds a
molecule that affects enzyme activity.
replication fork Y-shaped region of a replicating DNA molecule at which the
two daughter strands are formed and separate.
replication origin Location on a DNA molecule at which duplication of the
DNA begins.
replicative cell senescence Phenomenon observed in primary cell cultures as
they age, in which cell proliferation slows down and finally halts.
repressor Protein that binds to a specific region of DNA to prevent
transcription of an adjacent gene.
residue General term for the unit of a polymer. That portion of a sugar, amino
acid, or nucleotide that is retained as part of the polymer chain during the
process of polymerization.
respiration General term for a process in a cell involving the oxidative
breakdown of sugars or other organic molecules, and requiring the uptake of
O2 while producing CO2 and H2O as waste products.
respiratory chain Electron-transport chain in the inner mitochondrial
membrane that receives high-energy electrons derived from the citric acid
cycle and generates the proton gradient across the membrane that is used to
power ATP synthesis.
respiratory control Regulatory mechanism that controls the rate of electron
transport in the respiratory chain according to need via a direct influence of the
electrochemical proton gradient.
respiratory enzyme complex Any of the major protein complexes of the
mitochondrial respiratory chain that act as electron-driven proton pumps to
generate the proton gradient across the inner membrane.
resting membrane potential The membrane potential in equilibrium
conditions in which there is no net flow of ions across the plasma membrane.
restriction map Diagrammatic representation of a DNA molecule indicating
the sites of cleavage by various restriction enzymes.
restriction nuclease (restriction enzyme) One of a large number of nucleases
that can cleave a DNA molecule at any site where a specific short sequence of
nucleotides occurs. Extensively used in recombinant DNA technology.
restriction point Important checkpoint in the mammalian cell cycle. Passage
through the restriction point commits the cell to enter S phase. It corresponds
to Start in the yeast cell cycle.
retrotransposon Type of transposable element that moves by being first
transcribed into an RNA copy that is then reconverted to DNA by reverse
transcriptase and inserted elsewhere in the chromosomes.
retrovirus RNA-containing virus that replicates in a cell by first making a
double-stranded DNA intermediate.
reverse genetics Approach to discovering gene function that starts from the
DNA (gene) and protein and then creates mutants to analyze the gene's
function.
reverse transcriptase Enzyme first discovered in retroviruses that makes a
double-stranded DNA copy from a single-stranded RNA template molecule.
rhodopsin G-protein-linked light-sensitive receptor protein in the rod
photoreceptor cells of the retina.
ribonuclease Enzyme that cuts an RNA molecule by hydrolyzing one or more
of its phosphodiester bonds.
ribonucleic acid see RNA
ribosomal RNA (rRNA) Any one of a number of specific RNA molecules
that form part of the structure of a ribosome and participate in the synthesis of
proteins. Often distinguished by their sedimentation coefficient, such as 28S
rRNA or 5S rRNA.
ribosome Particle composed of ribosomal RNAs and ribosomal proteins that
associates with messenger RNA and catalyzes the synthesis of protein.
ribozyme RNA with catalytic activity.
RNA (ribonucleic acid) Polymer formed from covalently linked
ribonucleotide monomers.
RNA editing Production of a functional mRNA by insertion or alteration of
individual nucleotides in an RNA molecule after it is synthesized.
RNA interference (RNAi) Selective intracellular degradation of RNA that is
intended to remove foreign RNAs, such as those of viruses. Fragments cleaved
from free double-stranded RNA direct the degradative mechanism to other
similar RNA sequences. Widely exploited in a technique used to silence the
expression of selected genes.
RNA polymerase II holoenzyme Large pre-assumbled complex of RNA
polymerase II, most of the general transcription factors required for its
function, and the mediator protein complex.
RNA polymerase Enzyme that catalyzes the synthesis of an RNA molecule on
a DNA template from nucleoside triphosphate precursors. (6 8.)
RNA primer Short stretch of RNA synthesized on a DNA template. It is
required by DNA polymerases to start their DNA synthesis.
RNA processing control Control of gene expression by controlling how the
RNA transcript is spliced or otherwise processed.
RNA splicing Process in which intron sequences are excised from RNA
transcripts in the nucleus during formation of messenger and other RNAs.
RNAi see RNA interference
rod photoreceptor (rod) Photoreceptor cell type in the retina that is
responsible for noncolor vision in dim light.
rough endoplasmic reticulum (rough ER) Endoplasmic reticulum with
ribosomes on its cytosolic surface. Involved in the synthesis of secreted and
membrane-bound proteins.
rRNA see ribosomal RNA
rRNA gene Gene that specifies a ribosomal RNA (rRNA).