tain several proteins, that binds to centromeric DNA. and captures the microtubules that come from one. of the two spindle poles. The centromere of meta-. phase chromosomes is narrower than the regions dis-. tal to it, and therefore it is called the primary chro-. mosomal constriction. The centromere is generally. bordered by heterochromatin that contains repeti-. tious DNA (q.v.) and it is late to replicate. From a. structural standpoint, centromeres are of two major.
types, those that occupy a very small region ( 200. bp) of the chromosomal DNA and those that oc-. cupy large regions (40 kb to 5 mb). Saccharomyces. cerevisiae has centromeres of the first type (point. centromeres). Such small centromeres are expected,. since yeast chromosomes are 100 times smaller than. those from higher eukaryotes. The centromeres are. not chromosome-specific and function normally in. inverted sequence or when swapped between chro-.
mosomes. The minimal functional centromere is. only about 112 base pairs and occupies about 40 nm. of the B-form DNA. It is made up of three elements.. The central one, containing 88 kb, is about 93% AT.. The lateral elements have conserved sequences that. contain about 80% AT. Specific proteins bind to the. lateral elements and form a complex, which attaches. the chromosome to a single spindle microtubule.. Larger chromosomes have regional centromeres, which. bind 30 to 40 microtubules simultaneously. For ex-. ample, Drosophila centromeres contain 420 kb of. DNA and are made up of simple repetitive DNA.
segments that are required for the special chromo-. somal organization at the centromere. There are also. AT-rich segments, which may function in microtu-. bule binding. Four transcriptionally active genes. have been mapped within the centromere of chro-. mosome 2 of D. melanogaster. In Arabidopsis thali-. ana (q.v.) the centromeres vary in length from 1.4. to 1.9 megabases and make up about 7% of the. chromosomal DNA. Within the centromere regions. are sequences of 180 base pairs that are repeated. hundreds of times on all five chromosomes. Se-. . . quences that resemble retroposons (q.v.) are abun-. dant in centromeric regions.
Crossing over is dramat-. ically suppressed within centromeres. There are. about 200 genes contained in Arabidopsis centro-. meres, but many of these may be inactivated. How-. ever, at least 50 are transcriptionally active. See Ap-. pendix C, 1903, Waldeyer; 1980, Clark and Carbon;. 1999, Copenhaver et al.; CENP-A, lamins, Luzula,. MAD mutations, meiosis, microtubule, mitosis, yeast. artificial chromosomes (YACs).. centromere interference the inhibitory effect of. the centromere upon crossing over in adjacent chro-. mosomal regions.. centromere misdivision See isochromosome.. centromeric coupling the forming of paired cen-. tromeres early in diplonema. At first a protein en-. coded by Zip 1 (q.v.) holds the centromeres of. homologous and nonhomologous chromosomes to-. gether indiscriminately. But as time passes, the num-. ber of homologous pairs increases, even though the. total number of coupled centromeres remains the. same. This observation suggests that the Zip 1 pro-. tein holds the centromeres together while chromo-. some homology is assessed. Then when correct.
pairing is achieved, synaptonemal complexes are. constructed between the homologs. See Gowen. crossover suppressor.. centromeric index the percentage of the total. length of a chromosome encompassed by its shorter. arm. For example, in human somatic cells during. metaphase, chromosomes 1 and 13 have centro-. meric indexes of 48 and 17, respectively. Therefore,. chromosome 1 is metacentric with its short arm oc-. cupying 48% of the total length of the chromosome,. and chromosome 13 is acrocentric with a short arm. that only occupies 17% of the total length.. centrosome a cytoplasmic region surrounding a.
pair of centrioles (q.v.), but devoid of a limiting. membrane. The dense material surrounding the. . . paired centrioles is called pericentriolar material or. the centrosome matrix. All eukaryotes possess. centrosomes, and during cell division centrosomes at. opposite poles of the cell initiate the growth of the. microtubules of the spindle apparatus. In the mitotic. cells of the higher plants, centrioles are absent from. the centrosomes, but the centrosomal matrix con-. tains the necessary microtubule organizing activities. for spindle formation. See cyclin-dependent kinease. 2 (Cdk2), microtubule organizing centers (MTOCs),. parthenogenesis, spindle pole body, tubulin.. Cepaea a genus of land snails belonging to the. family Helicidae. C. hortenses and C. nemoralis ex-. hibit extensive variation in color and ornamentation. of the shell with longitudinal bands. These species. have been extensively studied in the field and in lab-. oratory colonies by population geneticists.
cephalic designating the head or the anterior end. of an animal.. cephalosporin an antibiotic with structural simi-. larities to penicillin (q.v.). It has the advantage of. not causing allergic reactions in patients that are al-. lergic to penicillin and of being inert to penicillinases. (q.v.). See Appendix C, 1964, Hodgkin.
Cephalosporium a genus of molds of importance because of the cephalosporin antibiotics they pro- duce. Cercopithecoidea a superfamily of primates con- taining the Old World (African and Asian) monkeys, baboons, macaques, colobines, etc. A sister group to the Hominoidea (q.v.).
The divergence of the Cer- copithecoidea and Hominoidea took place about 30 million years ago. Cercopithecus aethiops the African green mon- key. A catarrhine primate with a haploid chromo- some number of 30. About 20 genes have been as- signed to nine different linkage groups. Monolayers of cultured African green monkey kidney cells are often used for growing viruses and mycoplasmas. cereal a cultivated grass whose seeds are used as food; for example, wheat, oats, barley, rye, maize, etc. cerebroside a molecule composed of sphingosine, a fatty acid, and a sugar; abundant in the myelin sheaths of nerve cells. certation competition for fertilization among elongating pollen tubes. ceruloplasmin a blue, copper protein present among the α2 globulins of the plasma.
Approxi- mately 95% of the circulating copper of human be- ings is bound to ceruloplasmin. Ceruloplasmin is made up of eight subunits, each of molecular weight 18,000. See antihemophilic factor, Wilson disease. cesium-137 a radioisotope of cesium with a half life of about 30 years. Generated during the explo- sion of certain nuclear weapons, it is one of the ma- jor sources of radiation contamination from fallout. cesium chloride gradient centrifugation See cen- trifugation separation. CFTR cystic fibrosis transmembrane-conductance regulator. See cystic fibrosis. C(3)G Gowen crossover suppressor (q.v.).
C genes genes that code for the constant region of immunoglobulin protein chains. See immunoglob- ulin. chaeta a bristle, especially of an insect. Chaetodipus intermedius the rock pocket mouse, a species of rodents living in rocky habitats in adja- cent deserts of Arizona, New Mexico, and Mexico. The genetic basis for adaptive melanism (q.v.) was first elucidated in this species. The color of the ro- dents matches their natural substrates, and so pro- vides camouflage. The color of the dorsal fur is con- trolled by the MC1R gene (q.v.), and mutations at this locus determine the relative amounts of black vs. yellow melanin present in the hair. As predation eliminated mice with coat colors that failed to match their surroundings, genotypes were selected that provided the appropriate crypsis.
See Appendix A, Chordata, Mammalia, Rodentia; Appendix C, 2003, Nachman, Hoekstra, and D’Agostino; mel- anin. chaetotaxy the taxonomic study of the bristle pat- tern of insects. Chagas disease a disease in humans caused by the parasite Trypanosoma cruzi. It is transmitted by bloodsucking bugs in the genera Rhodnius and Tria- toma and by infected blood transfusion. The symp- toms include swelling at the site of the vector’s bite, fatigue, and fever during the acute stages, to cardiac, liver, and gastrointestinal problems, and eventually, death.
This disease is estimated to affect 16-18 mil- lion people and is a major problem in Central America, South America, and Mexico. Darwin is thought to have contracted Chagas disease in South America, and as a result, spent the remainder of his life as a semi-invalid. The disease is named after Car- los Chagas, a Brazilian doctor, who first described it in 1909 and who later determined the life cycle of the parasite and identified the insects that transmit it.
See Glossina, Trypanosoma; Appendix C, 2005, El- Sayed et al. chain reaction a biological, molecular, or atomic process in which some of the products of the pro- cess, or energies released by the process, are instru- mental in the continuation or magnification of the process.
chain termination codon See stop codon. chain terminator a molecule that stops the exten- sion of a DNA chain during replication. See 2′,3′- dideoxyribonucleoside triphosphates. chalcones a group of pigments biogenetically re- lated to anthocyans (q.v.). Chalcones give yellow to orange colors to the flowers of composites (q.v.). chaperones eukaryotic proteins that help some nascent polypeptide chains fold correctly into their tertiary shapes, stabilizing and protecting them in the process, and/or preventing them from making premature or nonproductive intermolecular associa- tions. Note that a chaperone forms a complex with a second protein to facilitate its folding, but chaper- ones are not part of the mature structure. Some of these molecular chaperones are heat-shock proteins (q.v.).
Some chaperones may bind to nascent poly- peptide chains while they are being synthesized on ribosomes, and they may also help the polypeptide move out of the tunnel of 60S ribosomal subunit. Other chaperones may keep the polypeptide in an unfolded conformation as it is being translated. This facilitates subsequent passage across membranes, as when protein enters the endoplasmic reticulum or a mitochondrion.
Also called chaperonins or molecular chaperones. See prions. character any detectable phenotypic property of an organism; synonymous with phenotype, trait. character displacement the exaggeration of spe- cies markers (visual clues, scents, mating calls, court- ship rituals, etc.) or adaptations (anatomical, phys- iological, or behavioral) in sympatric populations relative to allopatric populations of related species. This phenomenon is attributed to the direct effects of natural selection intensifying allesthetic traits use- ful for species discrimination or for utilizing differ- ent parts of an ecological niche (thereby avoiding di- rect competition). See Appendix C, 1956, Brown and Wilson. character states a suite of different expressions of a character in different organisms. These different states are said to be homologs.
A character may have a minimum of two states (present/absent or primi- tive/derived) or have many states. Chargaff rule for the DNA of any species, the number of adenine residues equals the number of thymine residues; likewise, the number of guanines equals the number of cytosines; the number of pu- rines (A + G) equals the number of pyrimidines (T + C). See Appendix C, 1950, Chargaff. charged tRNA a transfer RNA molecule to which an amino acid is attached; also termed aminoacylated tRNA. charon phages a set of 16 derivatives of bacterio- phage lambda that are designed as cloning vectors. They were named by their originators (F. R. Blattner and 11 colleagues) after the old ferryman of Greek mythology who conveyed the spirits of the dead across the River Styx. chase See pulse-chase experiment. chasmogamous designating a plant in which fer- tilization takes place after the opening of the flower. See cleistogamous.
chDNA chloroplast DNA. cheating genes any genetic elements that tend to increase in a population by meiotic drive (q.v.) even if they confer no selective advantage or perhaps even if they are harmful to the organisms in which they are present. Compare with selfish DNA. See segrega- tion distortion. checkpoint any one of several points in the cell cy- cle at which the progression of the cell to the next stage can be halted until more suitable conditions prevail.
One major checkpoint is in G1, just before the start of the S phase; the other is in G2, just be- fore the entry into mitosis. See Appendix C, 1989, Hartwell and Weinert; cell cycle, cyclins, DNA dam- age checkpoint, MAD mutations, maturation promot- ing factor (MPF), spindle checkpoint, RAD9. Che´diak-Higashi syndrome (CHS) a hereditary disease of humans causing decreased pigmentation of the hair and eyes and the production of defective lysosomes in leukocytes and melanocytes. CHS is caused by mutations in the lysosomal trafficking reg- ulator gene (LYST) (q.v.) which is located at 1q 42.1-2. A similar syndrome occurs in mice, mink,