ecule is generally a substrate of an enzyme produced by sG1 or sG2. Thus the system is an inducible one, since synthesis of P1 and P2 proceeds only in the presence of E. See allosteric effect, constitutive muta- tions, cro repressor, derepression, inducible system, lac operon, lambda repressor, operon, repressible sys- tem, selector genes. regulatory sequence a DNA sequence involved in regulating the expression of the structural gene(s) in the common operon. Examples include attenuators, operators, and promoters. See gene, regulator gene. regulons a group of operons that are under the control of the same regulatory protein.
An example is the SOS regulon that is induced when there is DNA damage. The SOS regulon is under the control of a repressor protein encoded by the LexA gene. The LexA protein represses four operons (specifying RecA, UvrA, UvrB, and UvrC), all of which are in- volved in recombination and DNA repair. See RecA protein, SOS boxes. reiterated genes genes that are present in multi- ple copies that are clustered together on specific chromosomes. Ribosomal RNA genes, transfer RNA genes, and histone genes are examples of such tan- dem multigene families. rejection in immunology, destruction of a cell or tissue graft by the immune system of the recipient, directed against antigens on the graft that are foreign to the recipient. See Appendix C, 1914, Little; 1927, Bauer; 1948, Snell; histocompatibility molecules. relational coiling the loose plectonemic coiling of two chromatids about one another. The two chro- matids cannot separate until uncoiling is completed.
See plectonemic spiral. relative biological effectiveness the ratio of the doses of different ionizing radiations required to pro- duce the same biological effect. relative molecular mass (Mr) the mass of a mole- cule relative to the dalton (q.v.). Mr has no units and has replaced the term molecular weight in the recent chemical literature. relative plating efficiency the percentage of inoc- ulated cells that give rise to colonies, relative to a control where the absolute plating efficiency is arbi- trarily set as 100. See absolute plating efficiency. relaxation complex a group of three proteins tightly bound to some E. coli supercoiled plasmids that convert supercoiled DNA to a nicked open cir- cle. When heated or treated with alkali, proteolytic enzymes, or detergents, one of these proteins nicks one strand at a specific site, thereby relaxing the su- percoil to a nicked open circular form. During relax- ation, the two smaller proteins are released, but the largest protein becomes covalently attached to the 5′-P end of the nick. Nicking plays a role in transfer of the plasmid during conjugation. The site of the nick establishes the transfer origin.
relaxation of selection cessation of selection in an experimental situation. relaxation proteins See helix-destabilizing proteins. relaxed control See plasmid. release factors specific proteins that read termina- tion codons and cause the release of the finished polypeptide. releaser in ethology (q.v.), the particular physical attributes or behavior of one animal that stimulates another animal to perform a specific response. relic coil the relaxed spirals often seen in prophase chromosomes. These coils are believed to be left over from the tightly coiled condition that the chro- mosome maintained at the previous metaphase. relict surviving beyond others of a kind, as a spe- cies that persists in one region after becoming ex- tinct elsewhere or a surviving species of a group of which others are extinct. relief in photography, etc., to produce the effect of a third dimension in a two-dimensional image, as by the use of shadows. See shadow casting. rem roentgen equivalent man, the dose of ionizing radiation that has the same biological effect as one rad of x-rays. It is equal to rads × rbe (relative bio- logical effectiveness). See Sievert (Sv).
renaturation the return of a protein or nucleic acid from a denatured state to its native three-dimensional configuration. Renner complex a group of chromosomes (and the genes within them) that are distributed as a unit generation after generation. Such complexes are found in species belonging to the genera Oenothera and Rhoeo, for example, where whole sets of chro- mosomes are involved in a series of interchanges. At first meiotic metaphase, one sees rings of bivalents, rather than independent tetrads. reovirus a virus whose name is derived from respi- ratory and enteric orphan virus. Reoviruses have been found in humans, but their relation to any dis- ease is uncertain. The term “orphan” is used for vi-
ruses “without a disease.” They are one of the few viruses that contain double-stranded RNA. rep roentgen equivalent physical (q.v.). repair See DNA repair. repair synthesis enzymatic excision and replace- ment of regions of damaged DNA as when UV- induced thymine dimers are removed. See Appendix C, 1964, Setlow and Carrier, Boyce and Howard- Flanders; 1965, Clark and Margulies; cut-and-patch repair. rep DNA repetitious DNA (q.v.).
repeated epitope See sporozoite. repeated gene families synonymous with multi- gene families (q.v.). repeat-induced point mutation (RIP) a mecha- nism, first detected in Neurospora crassa, by which repetitive DNA sequences are inactivated through mutation. During the sexual reproductive cycle, CG to AT mutations are induced in duplicated se- quences, and this creates targets in the DNA of veg- etative cells that are subsequently methylated. DNA methylation (q.v.) prevents gene transcription, and therefore repeat-induced point mutation causes the epigenetic silencing of repetitive DNA sequences. RIP is a process unique to fungi, and it may have evolved as a defense against transposable elements (q.v.). repeating unit the length of a nucleotide sequence that is repeated in a tandem cluster. repeats small tandem duplications (q.v.). repetition frequency the number of copies of a given DNA sequence present in the haploid ge- nome.
repetitious DNA nucleotide sequences that occur repeatedly in chromosomal DNA. For example, in the kangaroo rat, more than 50% of its genome contains three sequences—AAG, TTAGGG, and ACACAGCGGG—each repeated 1-2 billion times. Sequences of this sort are said to be highly repetitive. Sometimes such tandemly arranged, highly repeti- tive sequences are localized. For example, Drosoph- ila nasutoides has such repetitive sequences re- stricted to one of its four chromosomes. Highly repetitive sequences are also localized in constitutive heterochromatin (q.v.). There are two classes of dis- persed, highly repetitive DNA. They are abbreviated SINEs and LINEs, for Short and Long INterspersed Elements. SINEs are usually shorter than 500 base pairs and occur in 105-106 copies. In humans, the Alu family (q.v.) is a SINE. LINEs are DNAs longer than 5 kilobases and present in at least 104 copies per genome. The human genome contains one fam- ily of LINEs (LI). Retrotransposons (q.v.) are found in this family. Middle repetitive DNA consists of seg- ments 100-500 base pairs in length repeated 100 to 10,000 times each. This class of rep DNA contains the genes transcribed into rRNAs and tRNAs. See Appendix C, 1966, Waring and Britten; 1968, Brit- ten and Kohne; 1970, Pardue and Gall; 1978, Fin- negan et al.; 1988, Kazazian et al.; C value paradox, Dipodomys ordii, X-chromosome inactivation.
repetitive DNA repetitious DNA (q.v.). repetitive genes See multigene families. replacement sites positions within a gene at which point mutations alter the amino acid specification. replacement vector See lambda cloning vehicle. replica plating a technique used to produce iden- tical patterns of bacterial colonies on a series of petri plates. A petri plate containing bacterial colonies is inverted, and its surface is pressed against a cylindri- cal block covered with velveteen. In this way, 10 to 20% of the bacteria are transferred to the fabric. Subsequently, bacteria-free plates are inverted and pressed against the velveteen disc to pick up samples of the colonies.
About eight replicas may be printed from a single pad in this way. If the medium con- tained in each of the secondary plates differs in its selective properties, then the hundreds of different bacterial clones transferred may be scored simulta- neously for their responses to a given agent. See Ap- pendix C, 1952, Lederberg and Lederberg. replicase a nucleotide polymerase that replicates DNA or RNA molecules. In nature, DNA replicases require an RNA primer. RNA replicases do not re- quire primers, and this is one reason for speculating that the first replicating molecule was RNA, not DNA. See DNA polymerase, polymerase chain reac- tion, primer RNA, replication of DNA, RNA-dependent RNA polymerase. replication a duplication process requiring copy- ing from a template. replication-defective virus a virus defective in one or more genes essential for completing the infec- tive cycle. replication eye (bubble) the appearance under the electron microscope of a portion of a DNA mol- ecule that is undergoing replication. The eye-shaped region is formed by bidirectional replication forks
Replication of DNA
growing away from the origin. See D loop, replication of DNA. replication fork See replication of DNA. replication of DNA during DNA replication the two strands of the duplex molecule separate to form a replication fork. DNA polymerase (q.v.) then adds complementary nucleotides starting at the 3′ end. The strand that is continuously replicated in this way is referred to as the leading strand. The other strand is replicated discontinuously in short pieces. These Okazaki fragments are 1,000-2,000 nucleo- tides long, and each is built by the extension of a priming RNA molecule about 10 bases long. The ex- tension proceeds from 5′ to 3′. The segments are synthesized one after the other, so that the left Oka- zaki fragment in the diagram is the one that was syn- thesized last. Subsequently, the RNA primers are re- moved, the gaps are filled, and nicks are sealed by DNA ligases (q.v.). Eventually, the lagging strand is extended until it reaches the 3′ end of the template strand. When the last RNA primer is removed, there will be a 3′ terminal overhang. It follows that genetic information will be lost during each replication cy- cle.
This is prevented by having the chromosomes capped with repetitive noncoding DNA. See Appen- dix C, 1968, Okazaki et al.; 1970, Smos and Inman; primase, replicon, replisome, telomerase, telomere, zygotene DNA. replication origin a nucleotide sequence at which DNA synthesis begins; termed an ori site. Circular bacterial genophores have a single ori site, whereas there are many ori sites on each eukaryotic chromo- some. See replicon. replication rate the speed at which deoxyribonu- cleotides are polymerized at a replication fork by DNA polymerases. In E. coli, with a single replica- tion fork, the replication rate is approximately 50,000 base pairs per minute or about 833 per sec- ond. Compare with transcription rate, translation rate. replicative forms double-stranded nucleic acid molecules seen at the time of the replication of sin- gle-stranded DNA and RNA viruses. replicator a DNA segment that contains a replica- tion origin (q.v.).
replicon a genetic element that behaves as an au- tonomous unit during DNA replication. In bacteria, the chromosome functions as a single replicon, whereas eukaryotic chromosomes contain hundreds of replicons in series. Each replicon contains a seg- ment to which a specific RNA polymerase binds and a replicator locus at which DNA replication com- mences. The polymerase makes an RNA primer called an initiator. See Appendix C, 1963, Jacob and Brenner; 1968, Huberman and Riggs; autonomously replicating sequences, DNA fiber autoradiography, ori site, primase. replicon fusion the joining of two complete repli- cating systems, mediated by a transposon. If two plasmids are joined (one of which carries a transpo- son), the process is called plasmid fusion. replisome a ribosome-sized particle that contains a complex of proteins which carry out the replica- tion of DNA. In E. coli it contains DNA polymerase III, a primosome (q.v.), a helicase (q.v.) that un- winds the DNA ahead of the polymerase, a gyrase (q.v.) that relieves supercoiling (q.v.) ahead of the replication fork, and single-stranded DNA-binding proteins that prevent the strands from reannealing. See DNA polymerase, replication of DNA. reporter gene a gene (R) that (1) can generate a product which is easy to visualize and (2) can re- spond to a regulatory signal meant for a second gene. It is this gene (X) that is of primary interest, but
there may be no convenient method of recording its activity. The tissue-specific expression of gene X can be monitored by letting the R gene intercept a regu- latory signal meant for X. R responds by synthesizing its product and thus “reports” the time and place of activation of X. The lac Z gene of E. coli has been used extensively as a reporter. Its product is beta ga- lactosidase. This enzyme normally breaks down lac- tose, but under the conditions of the experiment an analog of lactose is provided. When this compound (5-bromo-4-chloro-indolyl-β-D-galactoside) is cleaved by beta galactosidase, it yields 5-bromo-4-chloro- indigo, a blue dye.
The production of this blue pig- ment in a particular tissue during development re- ports the activation of the gene under study. See en- hancer trap, lac operon. rep protein a helicase (q.v.) identified in the rep mutant strain of E. coli that hydrolyzes ATP while forcing the strands of the DNA helix apart. representation See abundance. representational difference analysis (RDA) a technique used to detect DNA segments in one sam- ple (the tester pool) that are not in another (the driver pool). DNAs from each pool are fragmented by digestion with restriction endonucleases (q.v.), and the fragments are amplified by the polymerase chain reaction (q.v.).
The amplicons from one pool are then allowed to hybridize with those from the other, and the hybrid molecules are removed. This subtractive hybridization leads to the enrichment of fragments that are unique to the tester pool. Such a technique could be used to isolate cDNAs from genes specifically activated by certain hormones. repressible enzyme an enzyme whose rate of pro- duction is decreased when the intracellular concen- tration of certain metabolities is increased. repressible system a regulatory system in which the product of a regulator gene (the repressor) blocks transcription of the operon only if it first reacts with an effector molecule (called the repressing metabo- lite). Thus, mRNA synthesis occurs only in the ab- sence of the effector. See regulator gene. Compare with inducible system. repressing metabolite See repressible system. repression 1. the inhibition of transcription or translation when a repressor protein binds to an op- erator locus on DNA or to a specific site on a mRNA. 2. the cessation of synthesis of one or more enzymes when the products of the reactions they catalyze reach a critical concentration. repressor a protein (synthesized by a regulator gene) that binds to an operator locus and blocks transcription of that operon.
See cro repressor, lac repressor, lambda repressor, regulator gene. reproduction probability the average number of children of patients with a specific hereditary disease in relation to the average number of children of comparable individuals who do not have the heredi- tary disease. The reproduction probability is a mea- sure of the selective disadvantage of a hereditary dis- ease. See fitness. reproductive death the suppression of the prolif- erative ability of a cell that otherwise would divide indefinitely. See genetic death. reproductive isolation the absence of interbreed- ing between members of different species. See isolat- ing mechanisms. reproductive potential the theoretical logarithmic rate of population growth when unimpeded by envi- ronmental limitations; also known as the biotic po- tential; symbolized r. See r and K selection theory. reproductive success for a given individual, the number of offspring that survive to reproduce.
See fitness. repulsion See coupling, repulsion configurations. ReqQ DNA helicase See helicase. RER rough endoplasmic reticulum. (q.v.). RES reticuloendothelial system (q.v.). rescue the restoration of a defective cell or tissue to normal development through such means as cyto- plasmic transplantation or transformation (q.v.) with a normal gene. residual genotype background genotype (q.v.). residual homology in species hybrids, the homol- ogy remaining between those chromosomes that have been derived from a common ancestral chro- mosome and have since diverged through the accu- mulation of mutations. See homoeologous chromo- somes. residue in biochemistry, referring to a small sub- unit that forms a compoment of a larger molecule. Thus, a protein digested by a protease yields amino acid residues, and nucleases release nucleotide resi- dues from nucleic acids. resistance factor a class of episomes that confer antibiotic resistance to the recipient bacterium. See R plasmid.