27 Mar

and are interlinked by a DNA polymerase. A DNA ligase joins the two ends of the “patch” to the broken strand to complete the repair. See AP endonuclease, repair synthesis, thymine dimer, xeroderma pigment- osum. cuticle the chitinous, acellular outer covering of insects. Cu Zn SOD See superoxide dismutase. C value the amount of DNA that comprises the haploid genome for a given species.

Diploid cells that result from fertilization have the 2C value until they enter the S phase of their cell cycle (q.v.). Fol- lowing S, they will have the 4C amount until mitosis produces two sibling nuclei, each with 2C. In species where females are XXAA and males XYAA (A = one set of autosomes), the diploid nuclei of females usually contain more DNA than male nuclei because the X chromosome has more DNA than the Y.

In Drosophila melanogaster, for example, measurements reported in 1980 by P. K. Mulligan and E. M. Rasch show that male nuclei have about 90% the amount of DNA contained in female nuclei. The genome sizes published for most organisms do not differenti- ate separate values for the two sexes. The table illus- trates the large range in the C values found among multicellular organisms. See Appendix C, 1948, Boi- vin, Vendrely, and Vendrely; 1950, Swift; Appendix F; cell cycle, chromosome set, C value paradox, ge- nome size. C value paradox the paradox that there is often no correlation between the C values of species and their evolutionary complexity. For example, the C values for mammals fall into a narrow range (between 2 and 3 pg).

By contrast, the C values for amphibia vary from 1 to 100 pg. However, the minimum C values reported for species from each class of eukary- otes does increase with evolutionary complexity. In species with C values above the expected range, there is a greater amount of noncoding DNA. Much of this DNA is repetitive and may result from the replication of transposable elements (q.v.).

See Appendix C, 1971, Thomas; chromatin diminution, repetitious DNA, selfish DNA, skeletal DNA hypothesis. CVS chorionic villi sampling (q.v.). cyanelles organelles that allow glaucocystophytes (q.v.) to perform photosynthesis. Cyanelles occupy an intermediate level of symbiotic integration be- tween free-living cyanobacteria (q.v.) and chloro- plasts (q.v.). Both cyanobacteria and cyanelles con- tain chlorophyll a.

The genomes of cyanelles are about one-tenth the size of free-living cyanobacteria, but they are similar in size to the genomes of the chloroplasts of plants. The DNA genome in each cy- anelle is present in about 60 copies.

Unlike the situa- tion in plants, where the large subunit of RuBisCO is encoded by chloroplast genes and the small sub- unit by nuclear genes, both subunits are encoded by the cyanelle genomes. See ribulose-1, 5-bisphosphate carboxylase-oxygenase (RuBisCO), serial symbiosis theory.

Cyanidioschyzon merolae a red alga about 2 mµ in diameter that inhabits sulfate-rich hot springs (pH 1.5,45° C). The whole-genome shotgun (WGS) as- sembly (q.v.) method has been used to determine its nuclear genome. This contains 16,520,305 bp of DNA distributed among 20 chromosomes. The ge- nome is unique in that only 26 of its 5,331 genes contain introns. C. merolae has the smallest genome of all photosynthetic eukaryotes so far studied.

This protoctist also has the smallest set of rRNA genes known for any eukaryote. Each cell contains one mi- tochondrion and one chloroplast. Both organelles have had their DNAs sequenced, and the mitochon- drion contains 32,211 149,987 bp of DNA. See Appendix A, Protoctista, Rhodophyta; Appendix C, Matsuzaki et al.; Appendix F; division rings, dynamin. Cyanobacteria a phylum in the kingdom Eubac- teria (see Appendix A). The cyanobacteria produce oxygen gas, an ability that distinguishes them from

other photosynthetic bacteria. In the older litera- ture, these bacteria were misclassified as blue-green algae and placed in the phylum Cyanophyta. The ancestors of present-day cyanobacteria were the dominant life form in the Proterozoic era, and the oxygen they generated from photosynthesis caused a transformation some 2 billion years ago of the earth’s atmosphere from a reducing to an oxidizing one. The serial symbiosis theory (q.v.) derives chlo- roplasts from cyanobacteria. See chlorophyll, cya- nelle, photosynthesis, Prochloron, stromatolites, Sy- nechocystis. cyanocobalamin cobalamin.

cyanogen bromide BrCN, a reagent used for split- ting polypeptides at methionine residues; commonly used in studies of protein structure and the determi- nation of amino acid sequences. cyanolabe See color blindness. cyanophage a virus that has a cyanobacterium as its host. Cyanophyta See Cyanobacteria. cyclically permuted sequences DNA sequences of the same length containing genes in the same linear order, but starting and ending at different positions, as in a circle.

For example, the genes ABCDEFG can be circularly permuted to give BCDEFGA, CDEFGAB, DEFGABC, and so forth. In T4 DNA, each phage contains a different cycli- cally permuted sequence that is also terminally re- dundant. Cyclic permutation is a property of a pop- ulation of phage DNA molecules, whereas terminal redundancy is a property of an individual phage DNA molecule. See headful mechanism, terminal re- dundancy. cyclical selection selection in one direction fol- lowed by selection in the opposite direction result- ing from cyclical environmental fluctuations, such as seasonal temperature changes.

If the generation time is short relative to the environmental cycle, different genotypes will be selected at different times, and the population will remain genetically inhomogeneous. cyclic AMP adenosine monophosphate with the phosphate group bonded internally to form a cyclic molecule; generated from ATP by the enzyme ade- nylcyclase; abbreviated cAMP. Likewise, guanosine monophosphate (GMP) can become a cyclic mole- cule by a phosphodiester bond between 3′ and 5′ atoms.

Cyclic AMP has been shown to function as an acrasin in slime molds and to be active in the reg- ulation of gene expression in both prokaryotes and eukaryotes. In E. coli, cyclic AMP is required for the transcription of certain operons. See Appendix C, 1957, Sutherland and Rall; adenylcyclase, catabolite repression, cellular signal transductions, CREBs, G pro- teins, protein kinase, second messenger. n

cyclins a family of proteins whose concentrations rise and fall during the cell cycle (q.v.). Cyclins form complexes with specific protein kinases, thereby ac- tivating them and regulating the passage of the cell through the cell division cycle. The protein kinases are called cyclin-dependent kinases (cdks) or cell- division cycle (cdc) kinases.

There are two main classes of cyclins: G1 cyclins, which bind cdks during G1 and are necessary for entry into the S phase, and mitotic cyclins, which bind cdks during G2 and sig- nal entry into mitosis. Mitotic cyclins are destroyed at the subsequent anaphase. Near their N-terminal ends, all cyclin proteins contain a destruction box.

This refers to a sequence of amino acids that deter- mines whether or not the cyclin will be degraded at anaphase. Cyclins are posttranslationally modified by the covalent attachment of multiple copies of ubiquitin (q.v.) to a lysine residue to the right of the destruction box.

Polyubiquitin-containing proteins are degraded by large protein complexes called pro- teasomes. The attachment of ubiquitin to mitotic cyclins requires the enzyme ubiquitin ligase and a rec- ognition protein that attaches to the destruction box.

G1 cyclins combine with different kinases than do mitotic cyclins. The result is a start kinase, which induces chromosome replication. See Appendix C, 1983, Hunt et al.; checkpoint, cyclin-dependent ki- nase 2 (Cdk2), maturation promoting factor (MPF), protein kinase. cycloheximide an antibiotic synthesized by Strep- tomyces griseus. The drug inhibits translation on 80S ribosomes.

Therefore, it suppresses cytosolic protein synthesis without affecting the synthesis of proteins in mitochondria or chloroplasts. Protein synthesis in these organelles can be specifically inhibited by

Cystic fibrosis (CF)

chloramphenicol, erythromycin, or tetracycline. See ribosome, ribosomes of organelles.

cyclorrhaphous diptera  flies belonging to the sub- order Cyclorrhapha, which contains the most highly developed flies. It includes the hover flies, the dro- sophilids, house flies, blow flies, etc.

cyclosis cytoplasmic streaming.
cyclotron See accelerator.
cys cysteine (q.v.).

cysteine a sulfur-bearing amino acid found in bio- logical proteins. It is important because of its ability to form a disulfide cross-link with another cysteine, either in the same or between different polypeptide chains. See amino acid, cystine, insulin.

csyteine proteases proteolytic enzymes in which a cysteine residue resides in the catalytic domain and is required for enzymatic activity. These enzymes form four large superfamilies consisting of at least 30 families, each of which has evolutionarily conserved sequence domains.

Examples of cysteine proteases include papain, caspases, cathepsins, and various deubiquitinating enzymes (all of which See). cystic fibrosis (CF) the most common hereditary disease of Caucasians. In the United States, the fre- quency of homozygotes is 1/2,000, while heterozy- gotes make up about 5% of the population.

CF is a generalized multiorgan system disease arising from viscous mucous secretions that clog the lungs and di- gestive tract. The disease is inherited as an auto- somal recessive and is caused by mutations in a gene residing on the long arm of chromosome 7 in region 31-32. The CF gene is approximately 250 kilobases long, and its 27 exons encode a protein containing 1,480 amino acids.

This has been named the cystic fibrosis transmembrane-conductance regulator (CFTR). The CF gene is expressed predominantly in mucus- secreting epithelial cells, such as those of the submu- cosal glands of the bronchi, the salivary glands, the sweat glands, pancreas, testes, and intestines.

The CFTR functions as a channel for chloride ions. Prop- er chloride transport is necessary for diluting and flushing mucus downstream from mucus-secreting glands. Frameshift, missense, nonsense, and RNA splicing mutations have been isolated from victims of the disease. The most common mutation is ∆F508.

The abbreviation indicates that there is a de- letion (∆) of phenylalanine (F) at position 508. This mutation is present in 60-70% of the CF chromo- somes from North American Caucasians. A study of ∆F508 chromosomes in European families indicates that the mutation arose during paleolithic times in a population resembling the present-day Basques (q.v.). ∆F508 results in a temperature-sensitive defect in protein processing.

At 27°C the chloride channels are normal, but at 37°C transport of CFTR from the endoplasmic reticulum to the cell membrane never occurs. Therefore, Cl− channels cannot form, and a very severe form of CF results. The diagram of the CFTR molecule shows that the ∆F508 mutation re- sides in the first of two nucleotide-binding domains (NBDs). The regulatory domain (RD) is a region

Cystic fibrosis transmembrane-conductance regulator (CFTR)

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