cousin is a cousin once removed of his father’s or mother’s first cousin. See consanguinity, inbreeding. covalent bond a valence bond formed by a shared electron between the atoms in a covalent com- pound. See disulfide linkage, glycosidic bonds, high- energy bond, peptide bond, phosphodiester.
covariance a statistic employed in the computa- tion of the correlation coefficient between two vari- ables; the covariance is the sum of (x − x) (y − y) over all pairs of values for the variables x and y, where x is the mean of the x values and y is the mean of all y values. cpDNA chloroplast DNA. Also abbreviated ct- DNA. See chloroplast.
CPEB protein cytoplasmic polyadenylation element- binding protein. A protein first identified in Xenopus oocytes, where it activates dormant mRNAs by elon- gating their poly(A) tails. Homologs of this protein exist in humans, mice, flies, and marine mollusks. In Aplysia, a neuronal isoform of CPEB protein is expressed in central nervous system synapses and functions to regulate local protein synthesis at acti- vated synapses and to strengthen these synapses dur- ing long-term memory formation. This protein is re- markable in that it has prion-like properties in its biologically functional state and it is thought that this prion-like transformation is required for main- taining synaptic changes associated with long-term memory storage. See Appendix C, 2003, Si et al.; memory, prions. CpG island See DNA methylation.
Craniata the subphylum of the Chordata contain- ing animal species with a true skull. See Appendix A. Crassostrea virginica See Pelecypoda. creationism the belief that the universe and living organisms originate from specific acts of divine cre- ation, as in the biblical account, rather than by natu- ral processes such as evolution; another term for cre- ation science. See fundamentalism. CREBs The abbreviation for cyclic AMP response element binding proteins.
These proteins are re- quired for the consolidation of short-term memory into long-term memory. Genes encoding CREBs have been cloned in Drosophila. One of these genes, dCREB2, encodes a protein, dCREBa, that activates transcription of genes that enhance the ability of the fly to consolidate short-term memory into long-term memory. The isoform dCREBb inhibits this process. See Appendix C, 1982, Kandel and Schwartz; 1994, Tully et al., cyclic AMP.
Cretaceous the most recent of the Mesozoic peri- ods, during which the dinosaurs continued to diver- sify. The first angiosperms appeared during the late Cretaceous, and pollination interactions with insects developed. The first marsupials and placental mam- mals arose. At the end of the Cretaceous, there oc- curred the second most severe of all mass extinc- tions. About half of all animal families were wiped out. All of the dinosaurs became extinct. The conti- nents formed from Pangea were now widely sepa- rated. See continental drift, geologic time divisions, impact theory.
cretinism a stunting of bodily growth and mental development in humans due to a deficiency of thy- roid hormones. Hereditary cretinism, which is often accompanied by goiter (q.v.) and deafness, consists of a group of metabolic disorders that results in a failure in the formation of sufficient thyroxine and triiodothyronine. The defects include the inability of the thyroid gland to accumulate sufficient iodine, to convert it into organically bound iodine, and to couple iodotyrosines to form iodothyronines. All he- reditary defects in thyroid hormonogenesis are in- herited as autosomal recessives. See thyroid hor- mones.
Creutzfeldt-Jakob disease a fatal degenerative nervous disorder caused by infectious proteins called prions (q.v.). The prion protein PrP is a normal pro- tein of the nervous system, and it is encoded by a gene located in chromosome 20 p12-pter. The dis- ease is caused by a mutant form of the protein, which is infectious, and contains no detectable nu- cleic acid. The syndrome gets its name from the German physicians Hans G. Creutzfeldt and Alfons M. Jakob, who gave the first description of patients with the condition in 1920 and 1921, respectively. Cricetulus griseus the Chinese hamster.
The ro- dent is a favorite for cytogenetic studies because of its small chromosome number (N = 11). A total of about 40 genetic loci have been assigned to specific chromosomes. See CHO cell line. cri du chat syndrome See cat cry syndrome. Crigler-Najjar syndrome a defect in the metabo- lism of bilirubin (q.v.) caused by a recessive muta- tion in a gene located between q21 and 23 on hu- man chromosome 1. The condition was first described in 1952 by John Crigler and Victor Najjar. Patients lack hepatic bilirubin UDP-glucuronyl transferase. This enzyme functions to conjugate bili- rubin with glucuronic acid prior to biliary excretion. In the absence of the enzyme, excess bilirubin builds
up in all tissues causing jaundice, brain damage, and death. crisis period the time interval of a primary cell culture, following a number of cell divisions, during which most secondary progeny die even though cul- ture conditions are adequate to initiate a new pri- mary culture of low cell density from a fresh isolate. See Hayflick limit, tissue culture. criss-cross inheritance referring to the passage of sex-linked traits from mother to son and from father to daughter.
cristae elaborate invaginations of the inner mito- chondrial membrane. CRM cross-reacting material (q.v.). cRNA synthetic transcripts of a specific DNA mol- ecule or fragment made by an in vitro transcription system. This cRNA can be labeled with radioactive uracil and then used as a probe (q.v.). Cro-Magnon man Homo sapiens sapiens living in the upper Pleistocene. Cro-Magnon replaced the Neanderthal (q.v.) throughout its range. cro repressor the protein encoded by the cro regu- lator gene of lambda (λ) bacteriophage (q.v.). The cro gene lies alongside C1 gene, with its promoter to the immediate left.
During transcription the host transcriptase moves to the right. Movement of this enzyme can be blocked by the lambda repressor (q.v.), which binds to an operator that overlaps the cro promoter. The cro repressor contains 66 amino acids. The monomers associate in pairs to form the active repressor, which binds to DNA via a helix- term-helix motif (q.v.). The cro repressor is required for the virus to enter the lytic cycle (q.v.).
Since the cro repressor represses the lambda repressor, it is sometimes called an antirepressor. See Appendix C, 1981, Anderson et al.; regulator gene. cross in higher organisms, a mating between ge- netically different individuals of opposite sex.
In mi- croorganisms, genetic crosses are often achieved by allowing individuals of different mating types to con- jugate. In viruses, genetic crossing requires infecting the host cells with viral particles of different geno- types. The usual purpose of an experimental cross is to generate offspring with new combinations of parental genes.
See backcross, conjugation, dihybrid, E1, F1, I1, monohybrid cross, P1, parasexuality, test cross. cross-agglutination test one of a series of tests commonly employed in blood typing in which erythrocytes from a donor of unknown type are mixed with sera of known types. crossbreeding outbreeding (q.v.). cross-fertilization union of gametes that are pro- duced by different individuals.
Compare with self- fertilization. cross hybridization (molecular) hybridization of a probe (q.v.) to a nucleotide sequence that is less than 100% complementary. cross-induction the induction of vegetative phage replication in lysogenic bacteria in response to com- pounds transferred from UV-irradiated F+ to nonir- radiated F− bacteria during conjugation. crossing over the exchange of genetic material be- tween homologous chromosomes. Meiotic crossing over occurs during pachynema and involves the non- sister strands in each meiotic tetrad. Each exchange results in a microscopically visible chiasma (q.v.).
In order for proper segregation of homologs at the first meiotic division, each tetrad must have at least one chiasma. For this reason meiotic recombination is enhanced in very short chromosomes. Crossing over can also occur in somatic cells during mitosis. In suitable heterozygotes this may result in twin spots (q.v.). Exchange between sister chromatids can also occur and is a sensitive indicator of DNA damage caused by ionizing radiations and chemical muta- gens.
Sister chromatid exchanges normally do not re- sult in genetic recombination. See Appendix C, 1912, Morgan; 1913, Tanaka; 1931, Stern; 1931, Creighton and McClintock; 1961, Meselson and Weigel; 1964, Holliday; 1965, Clark; 1971, Howell and Stern; 1989, Kaback, Steensma, and De Jonge; 1992, Story, We- ber, and Steitz; genetic recombination, Holliday model, human pseudoautosomal region, meiosis, Rec A protein, site-specific recombination. crossing over within an inversion See inversion. cross-linking formation of covalent bonds be- tween a base in one strand of DNA and an opposite base in the complementary strand by mitotic poisons such as the antibiotic mitomycin C or the nitrite ion. cross-matching See cross-agglutination test.
crossopterygian a lobe-finned bony fish, one group of which was ancestral to the amphibians. See living fossil. crossover fixation the spreading of a mutation in one member of a tandem gene cluster through the entire cluster as a consequence of unequal crossing over. crossover region the segment of a chromosome lying between any two specified marker genes.
crossover suppressor a gene, or an inversion (q.v.), that prevents crossing over in a pair of chro- mosomes. The Gowen crossover suppressor gene (q.v.) of Drosophila prevents the formation of synap- tonemal complexes (q.v.). crossover unit a 1% crossover value between a pair of linked genes.
cross-pollination the pollination of a flower with pollen from a flower of a different genotype. cross-reacting material any nonfunctional pro- tein reactive with antibodies directed against its functional counterpart.
For example, some patients with classical hemophilia (q.v.) produce a CRM that reacts with anti-AHF serum, but this protein has lost its ability to take part in the blood-clotting process. cross-reaction, serological union of an antibody with an antigen other than the one used to stimulate formation of that antibody; such cross-reactions usu- ally involve antigens that are stereochemically simi- lar or those that share antigenic determinants.
cross reactivation See multiplicity reactivation. crown gall disease an infection caused by the soil-borne bacterium Agrobacterium tumefaciens (q.v.) that is characterized by tumor-like swellings (galls) that often occur on the infected plant at the stalk just above soil level (the crown).
The neoplas- tic outgrowths are composed of transformed cells which synthesize metabolites specifically used by the bacterium. Plants belonging to more than 90 dif- ferent families are susceptible to crown gall disease. See Appendix C, 1907, Smith and Townsend; pro- miscuous DNA, selfish DNA, Ti plasmid.
crozier the hook formed by an ascogenous hypha of Neurospora or related fungi previous to ascus de- velopment. The hook is formed when the tip cell of an ascogenous hypha grows back upon itself. Within the arched portion of the hypha, cell walls are subse- quently laid down in such a way that three cells are formed. The terminal cell of the branchlet is uninu- cleate, the penultimate one is binucleate, and the antipenultimate one is uninucleate.
Fusion of the haploid nuclei of different mating types in the pen- ultimate cell occurs, and it enlarges to form the as- cus in which meiosis occurs. CRP cyclic AMP receptor protein. See catabolite activating protein (CAP). cruciform structure a cross-shaped configuration of DNA produced by complementary inverted re- peats pairing with one another on the same strand instead of with its normal partner on the other strand. See palindrome.
cryostat a device designed to provide low-temper- ature environments in which operations (like sec- tioning frozen tissues) may be carried out under con- trolled conditions. crypsis those mechanisms which enable a species to remain hidden from its predators.
Cryptic color- ation is a form of camouflage that makes the species inconspicuous against its natural backgrounds. Be- havioral crypsis includes stealthy movements and unflinching inactivity when a predator is nearby. See adaptive melanism. cryptic coloration See crypsis.
cryptic gene a gene that has been silenced by a single nucleotide substitution, that is present at a high frequency in a population, and that can be reac- tivated by a single mutational event. cryptic prophage a prophage that has lost certain functions essential for lytic growth and the produc- tion of infectious particles.
However, these defective viruses still retain some functional genes, and there- fore they can rescue mutations in related bacterio- phages by recombining with them to generate viable hybrids. See Escherichia coli.
cryptic satellite a satellite DNA sequence that cannot be separated from the main-band DNA by density gradient ultracentrifugation. Cryptic satellite DNA can be isolated from the main-band DNA by its unique features (e.g., by the more rapid reanneal- ing of the highly repeated segments that constitute the satellite). cryptic species phenotypically similar species that never form hybrids in nature. See sibling species. cryptogam a spore-bearing rather than a seed- bearing plant.
In older taxonomy a member of the Cryptogamia, including the ferns, mosses, algae, and fungi. See phanerogam. cryptomonads a group of single-celled algae char- acterized by a “nucleomorph,” sandwiched between the membranes that surround the chloroplast. Cryptomonads are thought to have arisen hundreds
of millions of years ago by the fusion of a red algal symbiont and a biflagellated protozoan. The proto- zoan was the source of the conventional nucleus, whereas the nucleomorph is all that remains of the symbiont’s nucleus. It contains three minute linear chromosomes with telomeres and densely packed genes and is surrounded by a double envelope with characteristic pores. Nucleomorphs represent nuclei that have undergone the greatest genomic reduction in the history of eukaryotes,
See Appendix A, Protoc- tista, Cryptomonada; Appendix C, 1999, Beaton and Cavalier-Smith; C value paradox, serial symbiosis the- ory, skeletal DNA hypothesis. Cryptosporidium a genus of protozoan parasites that cause gastrointestinal diseases of medical and veterinary importance. These protoctists are placed in the same phylum as the malaria parasites. How- ever, they lack apicoplasts and have no second host, unlike Plasmodium (q.v.).
Cryptosporidia have com- plex life cycles with motile and non-motile forms in both asexual and sexual stages. They spend most of their lives within the epithelial cells of the gut or in its lumen. The infective phase of Cryptosporidium is a cyst that contains several haploid sporozoites en- closed in a thick capsule. The cysts are about 3 µm in diameter, are easily spread via water, are resistant to most chemical disinfectants, and can be removed from drinking water only by filtration. C.
parvum, the cause of cryptosporidosis in humans, has a ge- nome size of 9 million base pairs distributed among 8 chromosomes. See Appendix A, Protoctista, Api- complexa. Cryptozoic a synonym for Precambrian (q.v.).
crystallins a family of structural proteins in the lens of the vertebrate eye. However, some crystallins play an enzymatic role in other tissues. For example, in reptiles and birds a form of crystallin is found in heart muscle, where it functions as a lactic dehydro- genase. c-src a cellular gene, present in various verte- brates, that hybridizes with src, the oncogene of the Rous sarcoma virus (q.v.).
The c-src genes code for pp60c-Src proteins that resemble pp60v-src proteins in their enzymatic properties. CTCF protein a highly conserved and ubiquitous DNA binding protein of vertebrates. CTCF is an 82 kDa protein with 11 zinc fingers, and it binds to DNA segments that contain the sequence CCCTC. The CTCF protein functions to silence transcription by preventing enhancers from interacting with pro- moters of genes on the other side of domain bound- aries. See Appendix C, 2000, Bell and Felsenfield; H19, insulator DNAs. ctDNA chloroplast DNA. Also abbreviated cpDNA.
See chloroplast. C-terminus that end of the peptide chain that car- ries the free alpha carboxyl group of the last amino acid. By convention, the structural formula of a pep- tide chain is written with the C-terminus to the right. See translation. “C”-type particles a group of RNA viruses with similar morphologies under the electron microscope, having a centrally placed, spherical RNA-containing nucleoid. These viruses are associated with many sarcomas and leukemias. The “C” refers to “cancer.” Cucumis a genus of nearly 40 species including several of considerable economic importance, such as the cucumber (C. sativus) and the muskmelon (C. melo).
Considerable genetic information is avail- able for both these species. Cucurbita a genus of about 27 species, including 5 that are extensively cultivated: C. pepo, summer squash; C. mixta, cushaws: C. moschata, winter squash; C. maxima, Hubbard squash; and C. ficifolia, Malabar gourds. Most genetic information is available for C. pepo and C. maxima. Culex pipiens the most widely distributed species of mosquito in the world.
The genetics of insecticide resistance has been intensively studied in this spe- cies. Giant polytene chromosomes occur in the sali- vary gland and Malpighian tubule cells of larvae. cull to pick out and discard inferior animals or plants from a breeding stock. cultigen a plant that is known only under cultiva- tion and whose place and method of origin is un- known.
cultivar a variety of plant produced through selec- tive breeding by humans and maintained by cultiva- tion. See strain. curie the quantity of a radioactive nuclide disinte- grating at the rate of 3.700 × 1010 atoms per second. Abbreviated Ci. 1 Ci = 3.7 × 1010 Bq. cut a double-strand incision in a duplex DNA mol- ecule. Compare with nick. cut-and-patch repair repair of damaged DNA molecules by the enzymatic excision of the defective single-stranded segments and the subsequent synthe- sis of new segments. Using the complementary strand as a template, the correct bases are inserted