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that controls the response of CFTR to protein kin- ases (q.v.). There are two transmembrane domains (TMDs) where the protein folds back and forth, spanning the lipid bilayer of the cell membrane six times. Positively charged arginine and lysine mole- cules (indicated by pluses in the diagram) are essen- tial for the passage of anions through the pore. Mis- sense mutations that replace these with neutral amino acids also cause CF.

CF heterozygotes appear to be resistant to cholera, which may explain why the mutants like ∆F508 have been retained in hu- man populations. See Appendix C, 1989, Tsui et al., 1993; Tabcharani et al.; 1994, Morral et al., Gabriel et al.; ABC transporter, calnexin, cellular signal trans- duction, cholera, gene. cystine a derived amino acid formed by the oxida- tion of two cysteine thiol side chains, which join to form a disulfide covalent bond.

Such bonds play an important role in stabilizing the folded configura- tions of proteins. See cysteine, insulin, posttransla- tional processing. cystoblast See cystocyte divisions. cystocyte divisions the series of mitotic divisions which generate the nurse cell/oocyte clones that characterize insects with polytrophic meroistic ova- ries (like Dorsophila). In D. melanogaster two or three stem-line oogonia reside in each germarium (q.v.).

Each stem cell (S) divides into two daughter cells. One behaves like its parent, and the other differenti- ates into a cystoblast (Cb). This cell, by a series of

four mitoses (M1-M4), each followed by incomplete cytokinesis, produces a branching chain of 16 inter- connected cells. In the diagram here, cystocytes (represented by open circles) belong to the first, sec- ond, third, or fourth generation. The area in each circle is proportional to the volume of the cell.

The number of lines connecting any two cells shows the division at which the ring canal (q.v.) joining them was formed. Cells 14 and 24 enter the oocyte devel- opmental pathway and form synaptonemal com- plexes (q.v.). These cells are therefore called pro- oocytes (q.v.).

See insect ovary types, polyfusome, stem cell. cytidine See nucleoside. cytidylic acid See nucleotide. cytochalasin B a mold antimetabolite that pre- vents cells from undergoing cytokinesis. See actin, contractile ring.

cytochromes a family of heme-containing proteins that function as electron donors and acceptors dur- ing the chains of reactions that occur during respira- tion and photosynthesis. Electron transport depends upon the continued oxidation and reduction of the iron atom contained in the center of the porphyrin prosthetic group (see heme).

The first cytochrome is thought to have arisen about two billion years ago, and the genes that encode cytochromes have been modified slowly by base substitutions since then. The cytochromes were the first group of proteins for which amino acid sequence data allowed the con- struction of an evolutionary tree. See Appendix C, 1963, Margoliash; 1967, Fitch and Margoliash.

cytoplasmic determinants

Cytochrome system

cytochrome system a chain of coupled oxidation/ reduction reactions that transports the electrons pro- duced during the oxidations occurring in the citric acid cycle (q.v.) to the final hydrogen and electron acceptor, oxygen, to form water. The molecules in- volved in this chain are NAD (q.v.), FAD (q.v.), co- enzyme Q (q.v.), and cytochromes b, c, a, and a3. The sequence of reactions is diagrammed above. See ATP synthase, electron transport chain, Leber heredi- tary optic neuropathy (LHON), Leigh syndrome, mito- chondrial proton transport. cytogamy synonymous with autogamy (q.v.).

cytogenetic map a map showing the locations of genes on a chromosome. cytogenetics the science that combines the meth- ods and findings of cytology and genetics. See sym- bols used in human cytogenetics. cytohet a eukaryotic cell containing two geneti- cally different types of a specific organelle; the term is an abbreviation for cytoplasmically heterozygous. For example, in the single-celled alga Chlamydomo- nas, the frequency of rare cytohets (containing chlo- roplasts from both parents) can be greatly increased by treatment of one parent (mating type +) with ul- traviolet light.

See mitotic segregation. cytokines a group of small proteins (5-20 kilo- daeltons) involved primarily in communication be- tween cells of the immune system. Unlike hormones of the endocrine system, which can exert their ef- fects over long distances, cytokines usually act lo- cally on nearby cells. The term includes interleukins, interferons, lymphokines, and tumor necrosis factors (all of which see).

Compare with autocrine. cytokinesis cytoplasmic division as opposed to karyokinesis (q.v.) See cleavage, contractile ring. cytokinins a family of N-substituted derivatives of adenine (q.v.) synthesized mainly in the roots of higher plants. Cytokinins phytokinins) promote cell division and the synthesis of RNA and protein. The first molecule with these properties was called kinetin (see Appendix C, 1956,

Miller). The first cytokinin obtained from a plant was zeatin. It was isolated from maize kernels in 1964.

cytological hybridization synonymous with in situ hybridization (q.v.). cytological map a diagrammatic representation of the physical location of genes at specific sites, gener- ally on dipteran giant polytene chromosomes or on human mitotic chromosomes.

cytology the branch of biology dealing with the structure, function, and life history of the cell. See Appendix C, 1838, Schleiden and Schwann; 1855, Virchow; 1896, Wilson. cytolysis the dissolution of cells. cytophotometry quantitative studies of the local- ization within cells of various organic compounds using microspectrophotometry.

Cytophotometric techniques are employed, for example, to determine changes in the DNA contents of cells throughout their life cycle. See Appendix C, 1936, Caspersson; microspectrophotometer. cytoplasm the protoplasm exclusive of that within the nucleus (which is called nucleoplasm). cytoplasmic asymmetry uneven distribution of cytoplasmic components in a cell. See cytoplasmic determinants, cytoplasmic localization.

cytoplasmic determinants molecules that are lo- calized in specific cytoplasmic regions of the unfer- tilized egg or zygote and affect cell fate decisions by segregating into different embryonic cells and con-trolling distinct gene activities in these cells. In the egg, such determinants are usually maternal mRNAs and proteins.

Cytoplasmic determinants are also found in some post-embryonic cells, where they produce cytoplasmic asymmetry (q.v.).

In dividing cells, this leads to asymmetric cell division in which each of the daughter cells differentiates into a differ- ent cell type. Also called localized cytoplasmic deter- minants or morphogenetic determinants. See bicoid, cytoplasmic localization, maternal effect gene, mater- nal polarity mutants, pole plasm.

cytoplasmic inheritance non-Mendelian heredity involving replication and transmission of extrachro- mosomal genetic information found in organelles such as mitochondria and chloroplasts or in intracel- lular parasites such as viruses; also called extranu- clear inheritance. See Appendix C, 1909, Correns and Bauer; mtDNA lineages.

cytoplasmic localization the process whereby maternally or zygotically synthesized molecules be- come situated in specific spatial locations in the egg or zygote. This has been most widely examined in the Drosophila egg (e.g., in formation of the pole plasm (q.v.) or in positioning of cytoplasmic deter- minants (q.v.) that are later required for embryonic body pattern formation) and thought to be a step- wise process involving synthesis of the maternal product, its transport to the desired location, an- choring, and maintenance of localization. These steps are dependent upon sequential gene expres- sion, cytoskeletal elements, and cell organelles. See Balbiani body, bicoid, maternal effect gene, maternal polarity mutants, mitochondrial cloud, sponge body.

cytoplasmic male sterility (CMS) pollen abortion due to cytoplasmic factors that are maternally trans- mitted, but that act only in the absence of pollen- restoring genes. Such sterility can also be transmit- ted by grafting. In maize, pollen death is due to “abortion proteins” secreted by mitochondria, and the genes required to restore pollen fertility lower the abundance of abortion proteins by reducing rates of transcription of their mRNAs. Hybrid corn seed is produced commercially by a breeding system in- volving CMS. Unfortunately, the abortion proteins also enhance susceptibility of the plants to fungal toxins.

See Appendix C, 1987, Dewey, Timothy, and Levings, Bipolaris maydis, hybrid corn. cytoplasmic matrix See microtrabecular lattice. cytoplast the structural and functional unit of an eukaryotic cell formed by a lattice of cytoskeletal proteins to which are linked the nucleus and the cy- toplasmic organelles. cytosine See bases of nucleic acids, 5-hydroxymeth- ylcytosine. cytosine deoxyriboside See nucleoside.

cytoskeleton an internal skeleton that gives the eukaryotic cell its ability to move, to assume a char- acteristic shape, to divide, to undergo pinocytosis, to arrange its organelles, and to transport them from one location to another. The cytoskeleton contains microtubules, microfilaments, and intermediate fil- aments. cytosol the fluid portion of the cytoplasm exclu- sive of organelles; synonymous with hyaloplasm. See cell fractionation.

cytostatic referring to any agent that suppresses cell multiplication and growth. cytotaxis the ordering and arranging of new cell structure under the influence of preexisting cell structure. The information controlling the three-di- mensional architecture of the eukaryotic cell is thought to reside in the structure of the cytoplasmic ground substance. Evidence for this comes from mi- crosurgical experiments on Paramecium.

Cortical segments reimplanted with inverted polarity result in a changed pattern that is inherited through hun- dreds of generations. See microtrabecular lattice. cytotoxic T lymphocyte a lymphocyte that binds to a foreign cell and kills it. Such lymphocytes recog- nize target cells on the basis of the antigenic proper- ties of their class I histocompatibility molecules. See helper T lymphocyte, T lymphocyte.

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