coevolution the evolution of one or more species in synchrony with another species as a consequence of their interdependence. Such a reciprocal adaptive evolution determines the patterns of host-plant utili- zation by insects. Also, parasites often evolve and speciate in harmony with their hosts.
Even parasitic DNA molecules—that is, lysogenic viruses and transposable elements hosts to prevent serious disruptions in the gene ac- tivities of the host that could reduce its fitness. See Fahrenholz’s rule, gene-for-gene hypothesis. cofactor a factor such as a coenzyme or a metallic ion required in addition to a protein enzyme for a given reaction to take place.
cognate tRNAs transfer RNA molecules that can be recognized by aminoacyl-tRNA synthetase en- zymes. cohesin a multi-subunit complex first identified in Saccharomyces cerevisiae that is essential for holding sister chromatids together from the time of DNA replication until the onset of anaphase. Cohesin is assembled from four subunits (Smc1, Smc3, Scc1, and Scc3).
Loss of function mutations in any of the genes that encode these proteins results in preco- cious dissociation of sister chromatids. There are high concentrations of cohesin in the pericentric re- gions of chromosomes.
The size of the pericentric cohesin-rich domains is the same for all chromo- somes. Cohesin is also found in intergenic regions of the arms of yeast chromosomes at intervals of about 11 kb. The chromosomal regions to which cohesin complexes bind contain DNA sequences that belong to the Alu family (q.v.).
During the anaphase stage of mitosis, the cohesin complexes joining the chro- matids are cleaved by separase (q.v.). Therefore the chromatids become independent chromosomes and can move to the opposite poles of the dividing cell.
During anaphase I of meiosis, the separase enzymes cleave the cohesins in the arms of the chromatids, so they are free to separate. However, the cohesins loaded on the centromeric DNA are protected by Sgo, and so the sister chromatids remain tethered at their centromeres. Sgo is degraded during telophase I, and therefore the centromeric cohesins are cleaved during anaphase II.
Now the sister chromatids are independent chromosomes and are drawn to oppo- site poles of the spindle. See meiosis, Sgo. cohesive-end ligation the use of DNA ligase to join double-stranded DNA molecules with comple- mentary cohesive termini that base pair with one an- other and bring together 3′-OH and 5′-P termini. Compare with blunt-end ligation.
cohesive ends See restriction endonuclease, sticky ends. cohort a group of individuals of similar age within a population. coilin a protein that is concentrated in Cajal bod- ies (q.v.) and serves as a cytological marker for this organelle.
Coilin shuttles between the nucleus and cytoplasm and may function as part of a transport system among the cytoplasm, Cajal bodies, and nucleoli. coincidence, coefficient of an experimental value equal to the observed number of double crossovers divided by the expected number.
coincidental evolution See concerted evolution. cointegrate structure the circular molecule formed by fusing two replicons, one possessing a transposon, the other lacking it. The cointegrate structure has two copies of the transposon located at both repli- con junctions, oriented as direct repeats. The forma- tion of a cointegrate structure is thought to be an obligatory intermediate in the transposition process.
The donor molecule (containing the transposon) is nicked in opposite strands at the ends of the transpo- son by a site-specific enzyme. The recipient mole- cule is nicked at staggered sites. Donor and recipient strands are then ligated at the nicks.
Each end of the transposon is connected to one of the single strands protruding from the target site, thereby generating two replication forks. When replication is complet- ed, a cointegrate structure is formed, which contains two copies of the transposon oriented as direct re- peats. An enzyme required in the formation of the cointegrate structure is called a transposase. The co- integrate can be separated into donor and recipient units each of which contains a copy of the transpo-
son. This process is called resolution of the cointe- grate, and it is accomplished by recombination be- tween the transposon copies. The enzyme involved in resolution is called a resolvase. coisogenic designating inbred strains of organisms that differ from one another only by a single gene as the result of mutation.
Contrast with congenic strains. coitus copulation; sexual intercourse in verte- brates. Colcemid trademark for a colchicine derivative ex- tensively used as a mitotic poison. colchicine an alkaloid that inhibits the formation of the spindle and delays the division of centro- meres.
Colchicine is used to produce polyploid vari- eties of horticulturally important species. It is also used in medicine in the treatment of gout. Also used to stop mitosis at metaphase (when chromosomes are maximally condensed) for preparation of karyo- types. See Appendix C, 1937, Blakeslee and Avery; Colchicum, haploid sporocytes, paclitaxel.
R NHCOCH3 for colchicine.
R NHCH3 for Colcemid.
Colchicum the genus of crocuses, including: C. autumnale, the autumn crocus, source of colchicine; C. aureus, the Dutch crocus; C. sativus, the Saffron crocus. cold-sensitive mutant a gene that is defective at low temperature but functional at normal tempera-ture.
coleoptile the first leaf formed during the germi- nation of monocotyledons. col factors bacterial plasmids that allow the cell to produce colicins (q.v.). colicin any of a group of proteins produced by cer- tain strains of E. coli and related species that have bactericidal effects. Colicinogenic bacteria are im- mune to the lethal effects of their own colicins.