Center of origin hypothesis
cell-mediated lympholysis the killing of “target”. cells by activated T lymphocytes through direct cell-. cell contact. Often used as an in vitro test of cell-. mediated immunity.. cell plate a semisolid structure formed by the co-. alescence of droplets that are laid down between the. daughter nuclei following mitosis in plants. The cell. plate is the precursor of the cell walls, and it is syn-. thesized by the phragmoplast (q.v.).. cell strain cells derived from a primary culture or.
cell line by the selection and cloning of cells having. specific properties or markers. The properties or. markers must persist during subsequent cultivation.. See in vitro marker, in vivo marker.. cell-surface receptors transmembrane proteins on. the surface of target cells. When they bind to appro-. priate extracellular signaling molecules, they are ac-. tivated and generate a cascade of intracellular signals. that alter the behavior of the target cells. Cell-sur-. face receptors are grouped into three classes: (1) re-. ceptors that are linked to ion channels, (2) receptors. linked to G proteins (q.v.), and (3) receptors linked. to enzymes. These enzymes are generally protein ki-. nases (q.v.). See ABC transporters, cellular signal. transduction, receptor-mediated endocytosis.. cell theory the theory that all animals and plants.
are made up of cells, and that growth and reproduc-. tion are due to division of cells. See Appendix C,. 1838, Schleiden and Schwann; 1855, Virchow.. cellular immunity immune responses carried out. by active cells rather than by antibodies. See Appen-. dix C, 1901, Mechnikov.. cellular signal transduction the pathways through. which cells receive external signals and transmit,. amplify, and direct them internally. The pathway. begins with cell-surface receptors (q.v.) and may end. in the cell nucleus with DNA-binding proteins that. suppress or activate replication or transcription. Sig-. naling pathways require intercommunicating chains.
of proteins that transmit the signal in a stepwise. fashion. Protein kinases (q.v.) often participate in. . . this cascade of reactions, since many signal transduc-. tions involve receiving an extracellular, chemical. signal, which triggers the phosphorylation of cyto-. plasmic proteins to amplify the signal. See ABC. transporter, cyclic AMP, gene-for-gene hypothesis,. G proteins, polycystic kidney disease, transforming. growth factor-β (TGF-β), Wnt..
cellular transformation See transformation.. cellulase an enzyme that degrades cellulose to glu-. cose.. cellulifugal moving away from the center of the. cell.. cellulose a complex structural polysaccharide that. makes up the greater part of the walls of plant cells.. As illustrated, cellulose is composed of a linear array. of beta-D-glucose molecules.. cell wall a rigid structure secreted external to the. plasma membrane. In plants it contains cellulose and. lignin; in fungi it contains chitin; and in bacteria it.
contains peptidoglycans.. cen See symbols used in human cytogenetics.. cenospecies a group of species that, when inter-. crossed, produce partially fertile hybrids.. Cenozoic the most recent geologic era, occupying. the last 65 million years and often called the age of. mammals. See geologic time divisions.. CENP-A centromeric protein A, a histone variant. that replaces H3 in centromeric nucleosomes.. CENP-A confers a unique structural rigidity to the.
nucleosomes into which it assembles.. center of origin an area from which a given taxo-. nomic group of organisms has originated and spread.. center of origin hypothesis the generalization. that the genetic variability is greatest in the territory. where a species arose. Conversely, marginal pop-. ulations are likely to show a limited number of. adaptations. Therefore, the regions where various.
agriculturally important plant species arose can. sometimes be identified by determining the amounts. of genetic polymorphism in different geographic. races. See Appendix C, 1926, Vavilov.. centimorgan See Morgan unit.. central dogma the concept describing the func-.
tional interrelations between DNA, RNA, and pro-. tein; that is, DNA serves as a template for its own. replication and for the transcription of RNA which,. in turn, is translated into protein. Thus, the direction. of the transmission of genetic information is DNA. → RNA → protein. Retroviruses (q.v.) violate this.
central dogma during their reproduction.. centric fusion breakage in the very short arms of. two acrocentric chromosomes, followed by fusion of. the long parts into a single chromosome; the two. small fragments are usually lost; also termed a Rob-.
ertsonian translocation or whole arm fusion. Centric. fusions are seen in newborn infants with a frequency. of 1 in 10,000. There is a marked excess of 21/21,. 13/14, and 14/21 translocations. Centric fusions are. an important cause of uniparental disomy. See Ap-. pendix C, 1911, Robertson; 1960, Polani et al.; dis-. omy, telomeric fusion site.
centrifugal acting in a direction away from the. center.. centrifugal selection See disruptive selection.. centrifugation separation any of various methods. of separation dispersions by the application of cen-. trifugal force. In the case of density gradient equilib-. rium centrifugation, a gradient of densities is established.
in a centrifuge tube by adding a high molecular. weight salt such as cesium chloride. The mixture of. molecules to be studied is layered in the surface of. the gradient and then centrifuged until each mole-. cule reaches the layer in the gradient with a buoyant. density equal to its own. In the case of density gradi-.
ent zonal centrifugation, the macromolecules are. characterized by their velocities of sedimentation. through a preformed sucrose gradient. In this case. . .
the sedimentation velocity is determined by molecu-. lar size and shape.. centrifuge an apparatus used for the separation of. substances by the application of centrifugal force. generated by whirling at a high rate of rotation a ves-. sel containing a fluid in which the substances are. suspended. See also ultracentrifuge.. centriole a self-reproducing cellular organelle gen-. erally consisting of a short cylinder containing nine. groups of peripheral microtubules (each group com-. posed of three fused microtubules) disposed about a. central cavity. Like DNA, centrioles replicate once. during the cell division cycle, but they do so conser-.
vatively by forming a completely new centriole. This. “daughter” centriole always lies at right angles to the. “mother,” and it grows outward until it reaches its. mature size. Centrioles are capable of movement. and always come to lie at the polar regions of the. spindle apparatus in dividing animal cells. The be-. havior of the centrioles is illustrated in the meiosis. entry. During anaphase the mother and daughter. centrioles separate, move apart, and go on to form. partner centrioles. Centrioles are required for animal. somatic cells to progress through G1 and into the S. phase of mitosis. The organelle that is ultrastructur-.
ally identical to the centriole forms the basal body. of a cilium. Centrioles do not occur in the cells of. higher plants. See Appendix C, 1888, Boveri; cell cy-. cle, centrosome, kinetosome, microtubule organizing. centers, p34 (CDC2).. centripetal acting in a direction toward the center.. centripetal selection See stabilizing selection.. centrolecithal egg one having centrally placed. yolk. See isolecithal egg, telolecithal egg.. centromere a region of a chromosome to which. spindle traction fibers attach during mitosis and mei-. osis. The position of the centromere determines. whether the chromosome will appear as a rod, a J,.
or a V during its poleward migration at anaphase. In. a very few species the traction fibers seem to attach. along the length of the chromosome. Such chromo-. somes are said to be polycentric or to have a diffuse. centromere. A replicated chromosome consists of two. chromatids joined at the centromere region. Late in. prophase, kinetochores develop on the two faces of. the centromere that point toward the spindle poles.. The microtubules of the traction fiber attach to the. kinetochores, as illustrated on page 69. In the older. literature, the terms centromere and kinetochore were. used synonymously. However, the kinetochore is. now defined as a complex structure, known to con-.