duction and functioning of chloroplasts is under the control of both nuclear genes and those of the organ- elle. Chloroplasts develop from protoplastids. These are small organelles surrounded by a double mem- brane. The inner one gives rise to a sparse internal membrane system from which the thylakoids de- velop. Chloroplasts are generally inherited uniparen- tally. Most angiosperms show maternal inheritance, whereas most gymnosperms show paternal inheri- tance of chloroplasts. Some chloroplast genes confer an advantage to the plants that possess them by en- coding proteins that immobilize herbicides.
See Ap- pendix C, 1837, von Mohl; 1883, Schimper; 1909, Correns and Bauer; 1951, Chiba; 1953, Finean et al.; 1962, Ris and Plaut; 1981, Steinbeck et al.; Appen- dix F; Chlamydomonas reinhardi, chloroplast DNA, chloroplast er, cyanelles, Prochloron, ribulose-1, 5- bisphosphate carboxylase-oxygenase, protein sorting, serial symbiosis theory, 5S RNA, Synechocystis. chloroplast DNA (chDNA) chloroplast DNAs are circular, like those of mitochondria, but many times longer. There are 40-80 DNA molecules per organ- elle. The DNA molecules form clusters in the stroma and are thought to attach to the inner mem- brane. The DNAs are devoid of histones. The chlo- roplast genome encodes all the rRNA and tRNA molecules needed for translation, plus about 50 pro- teins.
These include RNA polymerase, ribosomal proteins, components of the thylakoid membranes, and a family of proteins involved in oxidation-reduc- tion reactions. Some chloroplast genes contain in- trons. Chloroplast genomes do not contain trans- posons, and since chDNAs are usually transmitted uniparentally, there is no opportunity for recombi- nation. These facts may account for the observation that the protein-coding genes of chloroplasts evolve at a rate about five times slower than that of plant nuclear genes. For this reason chDNA variation has been extensively used in reconstructing plant phy- logenies.
See Appendix C, 1971, Manning et al.; 1972, Pigott and Carr; 1986, Ohyama et al., Shino- zaki et al.; 1987, Wolf, Li and Sharp; 1993, Hallick et al.; 2000, Lemieux, Otis and Turmel; 2004, Mat- suzaki et al. chloroplast ER See Chromista, Chrysomonads. chloroquine an anti-malarial drug that accumu- lates in the food vacuole of the merozoite (q.v.) and interferes with the detoxification of the heme (q.v.) released during the digestion of hemoglobin (q.v.). See artemisinin, hemozoin, malaria. chlorosis failure of chlorophyll development. CHO cell line A somatic cell line derived from Chinese hamster ovaries. The cells have a near dip- loid number, but over one-half of the chromosomes contain deletions, translocations, and other aberra- tions that have occurred during the evolution of the cell line. See Cricetulus griseus.
cholecystokinen a hormone secreted by the duo- denum that causes gallbladder contraction. cholera an epidemic disease caused by the bacte- rium Vibrio cholerae (q.v.). Each is a curved rod about 0.5 by 3 micrometers that possesses a polar flagellum. Vibrios are highly motile and spread in water contaminated with feces. If sufficient vibrios are ingested, some will reach the intestine and multi- ply. Virulent strains produce a toxin encoded by a chromosomal gene. The cholera toxin is made up of multiple polypeptide chains, and these are organized into A and B subunits. The B subunits bind to gan-
glioside receptors on the microvilli of the cells lining the intestine. B subunits, in groups of five, form a hydrophilic channel through which the A subunit enters the cell. It interacts with a specific G protein, converting it into a permanently active form.
This catalyzes reactions that stimulate secretion into the gut lumen of fluids rich in electrolytes. Therefore, the primary symptom of cholera is a profuse diar- rhea. The watery discharge may amount to 30 liters per day, and without treatment the victim will die in a few days of dehydration and the loss of ions, especially potassium and bicarbonate. Mutations causing cystic fibrosis (q.v.) may have been retained in human populations because of the resistance shown by heterozygotes to cholera. The CF gene en- codes a protein that functions as an ion channel.
Mutations at this locus therefore impair fluid trans- port and reduce the fluid loss transport and reduce the fluid loss induced by the cholera toxin. cholesterol the quantitatively predominant ste- roid of humans. A 27-carbon compound made up of a fused ring system. Cholesterol is a component of biomembranes and of the myelin sheaths that sur- round nerve axons. In insects, cholesterol serves as a precursor for ecdysone (q.v.).
See Appendix C, 1964, Hodgkin; adrenal corticosteroid, familial hypercholes- terolemia, vitamin D. cholinesterase See acetylcholinesterase. chondriome all the mitochondria of the cell re- ferred to collectively.
chondriosome a rarely used term, superceded by mitochondrion. chondroitin sulfuric acid a mucopolysaccharide commonly found in cartilage. chordamesoderm a layer of cells derived from the blastoporal lip that later in embryogenesis forms the mesoderm and notochord of the vertebrate embryo. The chordamesoderm acts as an organizer upon the overlying ectoderm to induce differentiation of neu- ral structures. See Spemann-Mangold organizer.
Chordata the phylum of animals with a noto- chord, a hollow dorsal nerve cord, and gill slits at some developmental stage. See Appendix A. chorea a nervous disorder, characterized by irregu- lar and involuntary actions of the muscles of the ex- tremities and face. See Huntington disease. chorioallantoic grafting the grafting of pieces of avian or mammalian embryos upon the allantois of the chick embryo under the inner shell-membrane. The implant becomes vascularized from the allan- toic circulation and continues development.