quantitative RT-PCR (reverse transcription–polymerase chain reaction) Technique in which a population of mRNAs is converted into cDNAs via reverse transcription, and the cDNAs are then amplified by PCR. The quantitative part relies on a direct relationship between the rate at which the PCR product is generated and the original concentration of the mRNA species of interest.
quaternary structure   Three-dimensional relationship of the different polypeptide chains in a multisubunit protein or protein complex.
quinone (Q) Small, lipid-soluble, mobile electron carrier molecule found in the respiratory and photosynthetic electron-transport chains. (Figure 14–17)
Rab cascade   An ordered recruitment of sequentially acting Rab proteins into Rab domains on membranes, which changes the identity of an organelle and reassigns membrane dynamics.
Rab effectors Molecules that bind activated, membrane-bound Rab proteins and act as downstream mediators of vesicle transport, membrane tethering, and fusion.
Rab proteins Monomeric GTPase in the Ras superfamily present in plasma and organelle membranes in its GTP-bound state, and as a soluble cytosolic protein in its GDP-bound state. Involved in conferring specificity on vesicle docking. (Table 15–5, p. 854)
Rac Member of the Rho family of monomeric GTPases that regulate the actin and microtubule cytoskeletons, cell-cycle progression, gene transcription, and membrane transport.
Rad51   Eukaryotic protein that catalyzes synapsis of DNA strands during genetic recombination. Called RecA in E. coli.
Ran (Ran protein) Monomeric GTPase of the Ras superfamily present in both cytosol and nucleus. Required for the active transport of macromolecules into and out of the nucleus through nuclear pore complexes. (Table 15–5, p. 854) rapidly inactivating K+ channel   Neuronal voltage-gated K+ channel, open when the membrane is depolarized, with a specific voltage sensitivity and kinetics of inactivation that induce a reduced rate of action potential firing at levels of stimulation only just above the threshold required, thereby resulting in a firing rate proportional to the strength of the depolarizing stimulus.
Ras   A small family of proto-oncogenes that are frequently mutated in cancers, each of which produces a Ras monomeric GTPase.
Ras (Ras protein) Monomeric GTPase of the Ras superfamily that helps to relay signals from cell-surface receptor tyrosine kinase receptors to the nucleus, frequently in response to signals that stimulate cell division. Named for the ras gene, first identified in viruses that cause rat sarcomas. (Figure 3–67)
Ras superfamily Large superfamily of monomeric GTPases (also called small GTP-binding proteins) of which Ras is the prototypical member. (Table 15–5, p. 854)
Ras-GAPs   Ras GTPase-activating proteins; increase the rate of hydrolysis of bound GTP by Ras, thereby inactivating Ras.
Ras-GEFs Ras guanine nucleotide exchange factors; stimulate the dissociation of GDP and the subsequent uptake of GTP from the cytosol, thereby activating Ras.
Ras–MAP-kinase signaling pathway Intracellular signaling pathway that relays signals from activated receptor tyrosine kinases to effector proteins in the cell including transcription regulators in the nucleus.
Rb gene The gene that is defective in both copies in individuals with retinoblastoma; its protein product plays a central role in cell-cycle control.
reading frame   The phase in which nucleotides are read in sets of three to encode a protein. An mRNA molecule can be read in any one of three reading frames, only one of which will give the required protein. (Figure 6–49)