Breakage-fusion-bridge cycle 2
C 1. Celsius (also Centigrade). 2. carbon. 3. the haploid amount of DNA. See C value, C paradox. 4. cytosine or cytidine. 14C a radioactive isotope of normal carbon (12C) emitting a weak beta particle. The half-life of 14C is 5,700 years. This radioisotope is extensively used as a tracer in molecular biology. CAAT box part of a conserved DNA sequence of about 75 base pairs upstream from the initiator for eukaryotic transcription; possibly involved in bind- ing RNA polymerase II. See Hogness box.
cadastral genes genes that restrict the action of other genes to specific regions of the organism. An example of such a boundary-setting gene is SUPER- MAN in Arabidopsis thaliana (q.v.). Flowers that contain inactive alleles of SUPERMAN have sta- mens in whorl 4. Since stamens require activities B and C (see floral identity mutations), these abnormal flowers suggest that genes capable of producing B are normally inhibited in whorl 4 by cadastral genes like SUPERMAN.
See floral identity mutations. cadherins glycoproteins composed of amino acids that function as cell-cell adhesion mole- cules. The N-terminal end of the molecule projects from the membrane surface and contains Ca2+ bind- ing sites. The C-terminal tail binds to the actin of the cytoskeleton. In between is a segment that func- tions as an integral part of the cell membrane. E- cadherins are the best characterized of the cadher- ins. They are present in many types of epithelial cells and are usually concentrated in the adhesion belts that hold the cells together. See cell-cell adhesion molecules (CAMs).
Caenobacter taenospiralis See killer paramecia. Caenorhabditis databases See Appendix E. Caenorhabditis elegans a small nematode whose developmental genetics has been extensively investi- gated. The worm is about 1 mm in length, and its life cycle, when reared at 20°C, is 3.5 days. Its trans- parent cuticle allows the visualization of every cell. The adult has 816 somatic cells, of which 302 are neurons. The complete lineage history and fate of every cell is known. C. elegans normally reproduces as a self-fertilizing hermaphrodite, which has two X chromosomes per cell, plus five pairs of autosomes. Loss of an X by meiotic nondisjunction leads to the production of males. These arise spontaneously among the progeny of hermaphrodites at a frequency of about 0.2%.
The mating of hermaphrodites with males made genetic analysis possible. The C. elegans genome contains 100 million base pairs and about 19,100 protein-coding genes. Exons and introns each make up about 24% of the genome, and each gene has an average of 5 introns. About a quarter of the genes are organized into operons (q.v.). The ribosomal and the 5S RNA genes occur in tandom arrays in autosomes I and V, respectively. The sel-12 gene of Caenorhabditis is homologous to a gene in humans that confers susceptibility to Alzheimer dis- ease (q.v.).
See Appendix A, Animalia, Pseudocoelo- mata, Nematoda; Appendix C, 1974, Brenner; 1977, Sulston, and Horvitz; 1981, Chalfe and Sulston; 1983, Greenwald et al.; 1998, C. elegans Sequencing Consortium; 2000, Rubin et al., Fraser et al.; Appendix E, Individual Databases; apoptosis, cell lineage mu- tants, daf-2, helitron, Hox genes, Pangrellus redivius, RNA interference (RNAi), trans-splicing, Turbatrix aceti, zinc finger proteins.
caffeine a stimulant found in coffee and tea. The usual portion of these beverages contains about 100 mg of caffeine, making it the most common drug taken regularly by human beings. Caffeine is a pu- rine analog that is mutagenically active in microbial systems. See alkaloid, bases of nucleic acids, theobro- mine.
Cairns molecule See theta replication. Cajal body a nuclear organelle first identified in 1903 by the Spanish neurobiologist Santiago Ramon y Cajal in mammalian neurons and called by him the accessory body. In 1969 A. Monneron and W. Bern-
hard rediscovered these organelles within the inter- phase nuclei of mammalian liver cells and named them coiled bodies on the basis of their appearance in electron micrographs. Cajal bodies are now generally identified by immunofluorescence with specific anti- bodies against the protein coilin (q.v.), which is con- centrated in them. The giant nucleus of amphibian oocytes (the germinal vesicle) contains 50 to 100 large Cajal bodies.
All three eukaryotic RNA poly- merases are found in oocyte Cajal bodies, along with many factors involved in transcription and process- ing of all types of RNA (pre-mRNA, pre-rRNA, tRNA, etc.). Based on studies of oocytes, Gall et al. have suggested that Cajal bodies are sites for preas- sembly of the transcription machinery of the nu- cleus, much as nucleoli are sites for preassembly of the translation machinery (ribosomes). See Appendix C, 1999, Gall et al.; nucleolus, posttranscriptional processing, snurposomes, transcriptosomes. calciferol vitamin D (q.v.). calcium an element universally found in small amounts in tissues. Atomic number 20; atomic weight 40.08; valence 2 +; most abundant isotope 40Ca; radioisotope 45Ca, half-life 164d, radiation emitted-beta particles.
Extracellular calcium plays a role in blood clotting and maintaining the integrity of biological membranes. For example, calcium chloride treatment of bacteria makes them perme- able to plasmids. Internally, calcium activates a vari- ety of enzymes, especially protein kinases (q.v.). See Appendix C, 1972, Cohen et al. calico cat See tortoiseshell cat. Calliphora erythrocephala a large fly in which polytene chromosomes occur in the ovarian nurse cells in certain inbred lines. The banding pattern of these giant chromosomes has been compared with those of pupal trichogen cells (q.v.).
callus the cluster of plant cells that results from tissue culturing a single plant cell. calmodulin an intracellular calcium receptor pro- tein that regulates a wide spectrum of enzymes and cellular functions, including the metabolism of cy- clic nucleotides and glycogen. It also plays a role in fertilization and in the regulation of cell movement and cytoskeletal control, as well as in the synthesis and release of neurotransmitters and hormones.
Cal- modulin is a heat- and acid-stable, acidic protein with four calcium-binding sites. It is found in all eukaryotic cells and has a molecular weight of 16,700. It appears to be the commonest translator of the intracellular calcium message. See second messenger. calnexin an integral membrane protein of the en- doplasmic reticulum (q.v.). Calnexin is a chaperone (q.v.) that binds misfolded glycoproteins and targets them for subsequent degradation by a proteasome (q.v.).
The product of the delta F508 allele of cystic fibrosis gene is an example of a glycoprotein that in- teracts with this chaperone. See cystic fibrosis (CF). calyx the sterile, outer whorl of floral parts com- posed of sepals. cambium the lateral meristem of vascular plants. Cambrian the earliest period in the Paleozoic era. Representatives of most animal phyla are present in Cambrian rocks. Algae, sponges, and trilobites (q.v.) were abundant. The Cambrian ended with a mass extinction.
Seventy-five percent of all trilobite fami- lies and 50% of all sponge families died off. See geo- logic time divisions. Camelus the genus of camels including C. bactri- anus, the two-humped camel; and C. dromedarius, the one-humped camel, also called the dromedary. cAMP See cyclic AMP. Campbell model of lambda integration a hy- pothesis that explains the mechanism of integration of phage lamda into the E. coli host chromosome. According to the model, linear lambda DNA is first circularized. Then prophage integration occurs as a physical breakage and reunion of phage and host DNA molecules precisely between the bacterial DNA site for phage attachment and a corresponding site in the phage DNA.
See Appendix C, 1962, Campbell. canalization the existence of developmental path- ways that lead to a standard phenotype in spite of genetic or environmental disturbances. canalized character a trait whose variability is re- stricted within narrow boundaries even when the or- ganisms are subjected to disturbing environments or mutations. canalizing selection elimination of genotypes that render developing individuals sensitive to environ- mental fluctuations. cancer a class of diseases of animals characterized by uncontrolled cellular growth. See Appendix E; anti-oncogenes, Burkitt lymphoma, carcinoma, immu- nological surveillance theory, leukemia, lymphoma, malignancy, melanoma, metastasis, myeloma, neo- plasm, oncogene, oncogenic virus, p53, papilloma, sarcoma, teratocarcinoma, teratoma.