Radiation-induced chromosomal aberration
radiation units See Gray (Gy), rad, rem, Roentgen (R), roentgen equivalent physical (rep), Sievert (Sv). radical scavenger a molecule with a high affinity for free radicals. If a radical scavenger is added to a biological system prior to irradiation, it may act as a protective agent. radioactive decay the disintegration of the nu- cleus of an unstable nuclide accompanied by the spontaneous emission of charged particles and/or photons.
radioactive isotope an isotope with an unstable nucleus that stabilizes itself by emitting ionizing ra- diations. The use of radioisotopes in biology dates back to 1943 when the X-10 reactor at the Oak Ridge Laboratory in Tennessee started their com- mercial production. See autoradiography, labeled compound, radioimmunoassay, tritium. radioactive series a succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results. radioactivity the spontaneous disintegration of certain nuclides accompanied by the emission of one or more types of radiation, such as alpha particles, beta particles, and gamma photons. radioautograph autoradiograph (q.v.). radioautographic efficiency autoradiographic ef- ficiency (q.v.). radioautography autoradiography (q.v.). radiobiology a branch of biology that deals with the effects of radiation on biological systems.
It in- cludes radiation genetics (q.v.). radiogenic element an element derived from an- other element by atomic disintegration. radiograph a shadow image made on photo- graphic emulsion by the action of ionizing radiation. The image is the result of the differential attenua- tion of the radiation during its passage through the object being radiographed. A chest x-ray negative is a radiograph.
r and K selection theory
radioimmunoassay a highly sensitive technique for the quantitative determination of antigenically active substances that are present in very small amounts, such as hormones. The concentration of an unknown, unlabeled antigen is determined by com- paring its inhibitory effect on the binding of radioac- tively labeled antigen to specific antibody with the inhibitory effect of known standards. Symbolized RIA. See Appendix C, 1957, Berson and Yalow; io- dine. radiological survey the evaluation of the radiation hazards incident to the production, use, or existence of radioactive materials or other sources of radiation under a specified set of conditions. radiomimetic chemical a chemical that mimics ionizing radiations in terms of damage to nucleic acids.
Radiomimetic compounds include sulfur mus- tards, nitrogen mustards, and epoxides (all of which see). radioresistance the relative resistance of cells, tis- sues, organs, or organisms to the injurious action of radiation. Ultraviolet-resistant bacteria, for example, can excise ultraviolet-induced thymine dimers from their DNA. See Deinococcus radiodurans. radiotracer See labeled compound, radioactive iso- tope. radon the name used to refer to the many isotopes of element 86. Radon is an inert gas that is readily soluble in water.
All its isotopes are radioactive with short half-lives, and all decay with the emission of densely ionizing alpha particles (q.v.). While such particles are too weak to penetrate the skin, they are very dangerous when radon is ingested or inhaled. Radon is found in nature because it is continuously formed by the radioactive decay of the longer-lived, precursor elements uranium and thorium, which oc- cur in certain minerals. The most common radon isotope in human environment is 222Rn, which has a half-life of 3.8 days. Radiation from radon is respon- sible for over half of the average exposure to humans from ionizing radiation. RAG-1 and RAG-2 genes that synergistically acti- vate V(D)J recombination (q.v.). In the gnathos- tomes studies so far, these two genes are adjacent and are coordinately transcribed but only in lym- phatic tissue (q.v.). In most cases their ORFs do not contain introns. In humans, RAG-1 and RAG-2 are on the short arm of chromosome 11.
Coordinate ex- pression of RAG-1 and RAG-2 is regulated by ge- netic elements on the 5′ side of the RAG-2 gene. The proteins encoded by RAG-1 and RAG-2 coexist in a complex that resides in the periphery of the lymphocyte nucleus. The complex recognizes and binds to recombination signals that flank the V, D, and J segments of the Ig and TCR genes. Each re- combination signal consists of a row of seven specific bases (CACAGTC), then a spacer, and then a row of nine bases (ACAAAAACC). The spacer contains either 12 or 23 bases, but these show no consistent ordering of specific nucleotides. The RAG-1 protein contains a homeobox (q.v.) by which it binds to the recombination signal. The RAG transposon excises a donor DNA segment and inserts it into the maturing fusion product of the Ig gene. In gnathosomes, Ig and TCR genes must be assembled before they can be expressed, and RAG-1 and RAG-2 are transcribed only in B and T lymphocytes (q.v.).
Agnathans and invertebrates lack these molecules and cannot form antigen-specific lymphocytes. This suggests that soon after the divergence of jawed and jawless vertebrates a transposon inserted itself into the germ line of the gnathostome ancestor, and this transposon was the source of RAG-1 and RAG-2. See Appendix A, Chordata, Craniata, Agantha, Gnathostoma; Appen- dix C, 1990, Oettinger et al.; 1996, Spanopoulou et al.; immunoglobulin genes, recombination activating genes (RAGs), somatic recombination, T cell receptor genes, Tc1/mariner element. ramets buds that can detach from a plant or ani- mal and result in the asexual production of offspring genetically identical to each other and the parent. Ramets can also refer to the specific offspring pro- duced by asexual budding from a single ancestral or- ganism (the ortet). See modular organisms. Rana frogs of this genus have been used widely in research.
The leopard frog, R. pipiens, is the most common species bred in the laboratory, and many of its mutations have been recovered and analyzed. This was the species in which the first experiments were performed involving transfer of somatic diploid nuclei into enucleated eggs. Mutant strains are also available for R. sylvatica, R. esculenta, and R. tempo- raria. R. esculenta is the only anuran for which work- ing maps of the lamp-brush chromosomes are avail- able. See Appendix C, 1952, Briggs and King. r and K selection theory a theory in population ecology that attempts to establish whether environ- mental conditions favor the maximization of r (the intrinsic rate of natural increase) or of K (the carry- ing capacity of the environment). When populations can expand without food reserves limiting their growth, then r selection is in control. When food re- serves limit population size, K selection takes over, and increase in one genotype must be at the expense of another. Whereas r selection operates in ecologi-
cal situations where food reserves fluctuate drasti- cally, and species are favored that reproduce rapidly and produce large numbers of offspring. K selection operates in populations that are close to the environ- mental carrying capacity, and species are favored that reproduce slowly and generate a few offspring that are well adapted to a relatively stable environ- ment. random assortment See assortment. random cloning synonymous with shotgun cloning (q.v.).
random drift genetic drift (q.v.). random mating a population mating system in which every male gamete has an equal opportunity to join in fertilization with every female gamete, in- cluding those gametes derived from the same indi- viduals (if the species is monoecious or hermaphro- ditic); panmixis. The Hardy-Weinberg law assumes random mating. Contrast with mate choice. random primers randomly generated oligodeoxy- ribonucleotides, some of which anneal to comple- mentary sequences in the template nucleic acid and serve as primers in reactions involving reverse tran- scriptase. random sample a sample of a population selected so that all items in the population are equally likely to be included in the sample. random sampling error See experimental error, sampling error.
random sequencing synonymous with shotgun se- quencing (q.v.). random-X inactivation the method of dosage compensation (q.v.) found in eutherian mammals. See paternal-X inactivation. Raphanobrassica the classic example of a fertile allotetraploid, obtained from hybrids between the radish and cabbage. See Appendix C, 1927, Karpe- chenko. rapidly reannealing DNA, rapidly reassociating DNA repetitious DNA (q.v.). rapid-lysing (r) mutants mutants of T-even phage that enhance the rate at which E. coli host cells are lysed; on a bacterial lawn, r-plaques are larger than wild-type plaques (r+). See plaque. rare bases purines (other than adenine and gua- nine) and pyrimidines (other than cytosine and ura- cil) found in transfer RNA (q.v.). Formulas are shown on page 373. See bases of nucleic acids. rare earth any of the series of very similar metals ranging in atomic number from 57 to 71. See peri- odic table.
Rassenkreis See circular overlap, polytypic spe- cies. rat See Rattus. rat kangaroo See Potorous tridactylus. Rattus the genus of rats including R. norvegicus, the brown rat, and R. rattus, the black rat. The white laboratory rat is an albino form of R. norvegicus. The laboratory rat has 21 chromosome pairs including the sex chromosomes X and Y. The genome size is 2.75 gbp, and the estimated number of structural genes is 25,000. See Appendix A, Mammalia, Ro- dentia; Appendix E. Rauscher leukemia virus a virus from the plasma cells of leukemic mice by Frank Rauscher. This virus was the source of one of the first reverse transcriptases to be isolated. See Appen- dix C, 1970, Baltimore; retrovirus, RNA-dependent DNA polymerase. R bands See chromosome banding techniques. RB the symbol for the gene causing retinoblastoma (q.v.). RBC red blood cell. rbc genes genes that encode the components of the enzyme ribulose-1,5-bisphosphate carboxylase- oxygenase (q.v.).
The large subunits of the enzyme are encoded by rbcL genes; the small subunits by rbcS. In prokaryotes the rbcL and rbcS genes are part of the same operon. In most photosynthetic eukary- otes, rbcL genes are in the chloroplast genome and rbsS genes are in the nuclear genome. See photosyn- thesis. RBE relative biological effectiveness (q.v.). rDNA 1. in general, any DNA regions that code for ribosomal RNA components. 2. specifically, a tan- dem cluster of eukaryotic rRNA genes with a suffi- ciently atypical base composition to allow its isola- tion directly from sheared genomic DNA. In recent literature, rDNA is also used to refer to hybrid mole- cules formed by uniting two or more heterologous DNA molecules. To avoid confusion, the symbol rtDNA should be used for such recombinant DNA molecules and rDNA should be reserved for ribo- somal DNA. See Appendix C, 1967, Birnstiel.
rDNA amplification The genes for rRNA are pref- erentially replicated during oogenesis in amphibia. In Xenopus laevis, for example, there are rDNA repeats integrated into the chromosomes of the oocyte. However, there are 2 million DNA re- peats distributed among about 1,000 extrachromo- somal nucleoli that lie near the periphery of the nu- cleus of each diplotene oocyte. These amplified genes arose from single copies of the chromosomal rDNA, and during pachynema they replicated extra- chromosomally by a rolling circle (q.v.) mechanism. These extrachromosomal nucleoli function to tran- scribe the rRNAs stored in the growing oocyte.
Am- plification of rDNA also occurs commonly in insects with panoistic ovaries and in the macronuclei of pro- tozoa, such as Tetrahymena. See Appendix C, 1968, Gall, Brown and Dawid; insect ovary types, Miller trees, nucleolus, nucleolus organizer. reading the unidirectional process by which mRNA sequences are decoded (translated) into amino acid sequences (polypeptide chains). reading frame a nucleotide sequence that starts with an initiation codon, partitions the subsequent nucleotides into amino acid-encoding triplets, and ends with a termination codon. The interval be- tween the start and stop codons is called the open reading frame (ORF). If a stop codon occurs soon after the initiation codon, the reading frame is said to be blocked.