element is a mobile genetic element that, when in- serted into the target gene, causes it to become inac- tivated. The regulator gene maintains the mutational instability of the target gene, presumably by its ca- pacity to release the receptor element from the tar- get gene and thus return that locus to its normal function. See transposable elements. controlling gene a genomic sequence that can switch on or off the transcription of one or more separate structural genes (q.v.).
Also called a regulator gene (q.v.). convergence the evolution of unrelated species occupying similar adaptive zones, resulting in struc- tures bearing a superficial resemblance wings of birds and insects). convergent evolution See convergence. conversion See gene conversion. conversion factors for RNA and protein molecules the average relative molecular mass amino acid is 110, while the Mr for a nucleotide is 330. With these values one can estimate, for exam- ple, the approximate Mr of a protein containing a given number of amino acids or the number of nu- cleotides in a specific RNA with a known Mr value. Cooley anemia the most severe form of beta tha- lassemia (sometimes called thalassemia major). Few or no functional beta globin chains are made, and this results in a life-threatening anemia.
The patient requires lifelong blood transfusions, and these lead to an iron overload that requires chelation therapy. The eponym refers to Thomas Benton Cooley, an American pediatrician who first described the condi- tion in 1925. See Desferal, thalassemia. coordinated enzymes enzymes whose rates of production vary together. For example, the addition of lactose to the medium causes the coordinated in- duction of beta-galactosidase and beta-galactoside permease in E. coli. Such enzymes are produced by cistrons of the same operon. See regulator genes.
Cope law of the unspecialized the generalization put forward by the nineteenth-century paleontolo- gist Edward Drinker Cope that the evolutionary novelties associated with new major taxa are more likely to originate from a generalized, rather than from a specialized, member of an ancestral taxon. Cope size rule the general tendency for animals to increase in body size during the course of phyletic evolution (q.v.). copia elements retrotransposons (q.v.) of Dro- sophila existing as a family of closely related base se- quences that code for abundant mRNAs. There are usually between 20 and 60 copia elements per ge- nome.
The actual number depends on the Drosoph- ila strain employed. The copia elements are widely dispersed among the chromosomes, and the sites oc- cupied vary between strains. Each copia element ranges in size from 5 to 9 kilobase pairs, and it car- ries direct terminal repeats about 280 base pairs long. Several Drosophila mutations have been found that result from the insertion of copia-like elements. See Appendix C, 1985, Mount and Rubin. copolymer a polymeric molecule containing more than one kind of monomeric unit. For example, co- polymers of uridylic and cytidylic acids (poly UC) are used as synthetic messengers.
copper a biological trace element. Atomic number 29; atomic weight 63.45; valance 1+, 2+; most abun- dant isotope 63Cu; radioisotope 64Cu, half-life 12.8 hours, radiations emitted—gamma rays, electrons, and positrons. See Wilson disease. copy-choice hypothesis an explanation of genetic recombination based on the hypothesis that the new strand of DNA alternates between the paternal and maternal strands of DNA during its replication. Therefore crossing over involves a switch by the forming-strand between templates and does not re- quire physical breakage and reunion of exchanged strands.
copy DNA synonymous with complementary DNA. See cDNA. copy error a mutation resulting from a mistake during DNA replication. core 1. the region of a nuclear reactor containing the fissionable material. 2. synaptonemal complex. core DNA the segment of DNA in a nucleosome (q.v.) that wraps around a histone octamer. core granule RNP granules in the ommatidia of Drosophila. The xanthomatins and drosopterins are normally bound to these granules. core particle a structural unit of eukaryotic chro- mosomes revealed by digestion with micrococcal nuclease to consist of a histone octamer and a 146 base pair segment of DNA. See nucleosome.
corepressor in repressible genetic systems, the small effector molecule (usually an end product of a metabolic pathway) that inhibits the transcription of genes in an operon by binding to a regulator protein (aporepressor). Also called a repressing metabolite.
corm a swollen vertical underground stem base containing food material and bearing buds. It can function as an organ of vegetative reproduction. The crocus and gladiolus have corms. corn Zea mays, the most valuable crop plant grown in the United States, it ranks along with wheat, rice, and potatoes, as one of the four most important crops in the world.
Corn is generally clas- sified in five commercial varieties on the basis of ker- nel morphology: (1) Dent corn (var. indentata), the most common variety of field corn; the kernel is in- dented, from drying and shrinkage of the starch in the summit of the grain.
(2) Flint corn (var. indur- ata), the kernel is completely enclosed by a horny layer and the grain is therefore smooth and hard; flint corn is the fastest to mature.
(3) Sweet corn (var. saccharata), which is grown for human con- sumption, is picked when it is filled with a milky fluid, before the grain hardens.
(4) Popcorn (var. everta), the ear is covered with small kernels that are enclosed by a tough coat; when heated the con- tained moisture is turned to steam and the kernel explodes.
(5) Flour corn (var. amylacea), the ear contains soft, starchy kernels; it requires a long growing season and is therefore grown primarily in the tropics. See double cross, hybrid corn, pod corn, quantitative inheritance, teosinte, Zea mays. coronavirus a family of viruses that have a posi- tive-strand RNA genome and are characterized by a viral envelope from which petal-shaped spikes pro- trude.
The virus causing severe acute respiratory syndrome (SARS) in humans belongs to this family. Its genome contains 27,727 nucleotides. See Balti- more classification of viruses, enveloped viruses. corpus allatum an endocrine organ in insects that synthesizes the allatum hormone (q.v.). In the larvae of cyclorrhaphous diptera, the corpus allatum forms part of the ring gland (q.v.).
corpus cardiacum an endocrine organ in insects consisting of a central bundle of axons enveloped by cortical cells. Axons from the corpus cardiacum en- ter the corpus allatum. Most axons associated with the corpus cardiacum have their cell bodies in the pars intercerebralis.
The cortical cells and many of the axons contain numerous neurosecretory spheres (q.v.). corpus luteum a mass of yellowish tissue that fills the cavity left after the rupture of the mature ovum from the mammalian ovary. correction in a hybrid DNA sequence, the re- placement (i.e., by excision and repair) of illegiti- mate nucleotide base pairs by bases that pair prop- erly.
correlated response the change in one character occurring as an incidental consequence of the selec- tion for a seemingly independent character. For ex- ample, reduced fertility may accompany selection for increased bristle number in Drosophila. See plei- otropy. correlation the degree to which statistical vari- ables vary together. It is measured by the correlation coefficient (r), which has a value from zero (no corre- lation) to −1 or +1 (perfect negative or positive cor- relation, respectively). corresponding genes See gene-for-gene hypoth- esis.
corridor a migration route allowing easy dispersal for certain species. corticosterone one of a family of adrenal cortical hormones influencing glucose metabolism. corticotropin See adrenocorticotropic hormone. Corynebacterium diphtheriae the bacterium re- sponsible for diphtheria. This was the first disease shown to be caused by a toxin secreted by a bacte- rium and the first to be successfully treated by an antitoxin (q.v.).
See Appendix A, Bacteria, Actino- bacteria; Appendix C, 1888, Roux and Yersin; 1890, vonBehring; diphtheria toxin. COS cells a monkey cell line that has been trans- formed by an SV40 viral genome containing a defec- tive origin of viral replication. When introduced into COS cells, recombinant RNAs containing the SV40 origin and a foreign gene should replicate many copies. cosmic rays high-energy particulate and electro- magnetic radiations originating outside the earth’s atmosphere. cosmid plasmid vectors designed for cloning large fragments of eukaryotic DNA (i.e., in the size range of 40-45 kilobases).
The term signifies that the vec- tor is a plasmid into which phage lambda cos sites (q.v.) have been inserted. As a result, the plasmid DNA can be packaged in a phage coat in vitro. See Appendix C, 1977, Collins and Holm; genomic li- brary, physical map. cos sites cohesive end sites, nucleotide sequences that are recognized for packaging a phage DNA mol- ecule into its protein capsule. cot the point (symbolized by C0t1/2) in a reanneal- ing experiment where half of the DNA is present as double-stranded fragments; also called the half reaction time.
If the DNA fragments contain only unique DNA sequences and are similar in length, then C0t1/2 varies directly with DNA complexity (q.v.). See reassociation kinetics. cotransduction the simultaneous transduction of two or more genes because the transduced element contains more than one locus. cotransformation 1. the simultaneous transforma- tion of two or more bacterial genes; the genes co- transformed are inferred to be closely linked because transforming DNA fragments are usually small.
Also called double transformation. 2. in molecular biology, introduction of two physically unlinked sets of genes, one of which codes for a selectable marker, into a cell.
This technique is useful in animal cells in which the isolation of cells transformed with a gene that does not code for a selectable marker has been problematic.
cotranslational sorting See protein sorting. cotton See Gossypium. Coturnix coturnix japonica the Japanese quail, a small bird domesticated since the 12th century, and currently used as a laboratory animal. The European quail is Coturnix coturnix coturnix. See Appendix A, Aves, Neognathae, Galliformes. cot value See cot. cotyledon the leaf-forming part of the embryo in a seed.
Cotyledons may function as storage organs from which the seedling draws food, or they may absorb and pass on to the seedling nutrients stored in the endosperm. Once the cotyledon is exposed to light, it develops chlorophyll and functions photo- synthetically as the first leaf.
counteracting chromatographic electrophoresis a group of methods for purifying specific molecules from a mixture by the application of two counter- acting forces; specifically, the chromatographic flow of a solute down a separation column vs. solute elec- trophoresis in the opposite direction. countercurrent distribution apparatus an auto- mated apparatus used for separating mixtures.
The method takes advantage of differences in the solubil- ities of the components of the mixture in two im- miscible solvents. An example of the usefulness of the technique is in the separation of different trans- fer RNA molecules. counterselection a technique used in bacterial conjugation experiments to allow recovery of re- combinant F− cells, while at the same time selecting against (preventing growth of) Hfr donor cells.
For example, suppose the Hfr donor strain is susceptible to an antibiotic (such as streptomycin) and can syn- thesize histidine; the streptomycin locus must be so far from the origin of chromosome transfer that the mating pairs, which inevitably break apart, have sep- arated before the str locus has been transferred. Sup- pose further that the recipient F− cell cannot make histidine (his−) but is resistant to the antibiotic (Strr). Only His+Strr recombinants can survive on a me- dium lacking histidine and containing streptomycin.
The desired gene (His+ in this case) is called a selected marker; the gene that prevents growth of the male (strs in this case) is called the counterselective marker. coupled reactions chemical reactions having a common intermediate and therefore a means by which energy can be transferred from one to the other.
In the following pair of enzyme catalyzed re- actions, glucose-1-phosphate is the common inter- mediate that is formed in the first reaction and used up in the second: 1. ATP + glucose → ADP + glucose-1-phosphate 2. glucose-1-phosphate + fructose → sucrose + phosphate A molecule of sucrose is synthesized from glucose and fructose at the expense of the energy stored in ATP and transferred by glucose-1-phosphate.
coupled transcription-translation a characteristic of prokaryotes wherein translation begins on mRNA molecules before they have been completely tran- scribed. coupling, repulsion configurations when both nonallelic mutants are present on one homolog and the other homologous chromosome carries the plus alleles (a b/++), the genes are said to be in the cou- pling configuration.
The repulsion configuration re- fers to a situation in which each homolog contains a mutant and a wild-type gene (a+/+b). See cis-trans configurations. courtship ritual a characteristic genetically deter- mined behavioral pattern involving the production and reception of an elaborate sequence of visual, au- ditory, and chemical stimuli by the male and female prior to mating.
Such rituals are interpreted as en- suring that mating will occur only between the most fit individuals of the opposite sex and the same spe- cies. See allesthetic trait, gustatory receptor genes, mate choice, sexual selection. cousin the son or daughter of one’s uncle or aunt. The children of siblings are first cousins. Children of first cousins are second cousins. Children of second cousins are third cousins, etc. The child of a first