Circular dichroism (CD) A spectroscopic method that measures the difference in absorbance of left- and right-handed circularly polarized light by a material as a function of the wavelength. Most biological molecules, including proteins and nucleic acids, are chiral and show circular dichroism in their ultraviolet absorption bands, which can be used as an indication of secondary structure. Metal centers that are bound to such molecules, even if they have no inherent chirality, usually exhibit CD in absorption bands associated with ligand-based or ligand-metal charge-transfer transitions. CD is frequently used in combination with absorption and MCD studies to assign electronic transitions.
Cis In inorganic nomenclature, cis is a structural prefix designating two groups occupying adjacent positions. (The term is not generally recommended for precise nomenclature purposes of complicated systems.)
Cisplatin cis-Diamminedichloroplatinum(II). An anti-tumor drug highly effective in the chemotherapy of many forms of cancer. of major importance in the anti-tumor activity of this drug is its interaction with the nucleic acid bases of DNA.
Cistron A segment of DNA that codes for a single polypeptide domain; another name for a gene.
Cladistics A way to classify organisms by common ancestry, based on the branching of the evolutionary family tree. organisms that share a common ancestor and have similar features are put into groups called clades. At each diverging line, there are two branching lines of descendants, and evolution plays a role in future changes in characteristics.
Cladogenesis The evolutionary splitting of lineages; one or more new species comes from an existing parent species, i.e., speciation. Also called branching evolution.
Cladogram A pictorial representation of a branching tree that depicts species divergence from a common ancestry.
Class The taxonomic ranking of plants and animals that is between phylum and order.
Classical conditioning The presentation of two stimuli at the same point in time: a neutral stimulus and a conditioned stimulus; the changes in behavior arising from the presentation of one stimulus in the presence of another. The pairing leads to the neutral stimulus associating with the properties of the conditioned.
Cleavage The process of cell division in an early embryo. Initial stages in embryonic development where the zygote converts to a ball of cells through divisions of clearly marked blastomeres, usually from a succession from first through sixth cleavages (2-64 cells). Each species of organism displays a characteristic cleavage pattern that can be observed. Cleavage divides the embryo without increasing its mass.
Cleavage furrow A groove composed of actin-rich contractile microfilaments that draws in tight to separate daughter cells during cytokinesis. Also called the contractile ring.
Cleistogamous A flower that does not open and is self-pollinated. Pollen is transferred directly from the anthers to the stigma of the same flower.
Cleptoparasite The parasitic relation in which a female seeks out the prey or stored food of another female, usually of another species, and appropriates it for the rearing of her own offspring.
Climax The final stage in succession where the constituent species populations fluctuate normally instead of acting as replacements of other species. The constituent species will self-perpetuate as long as all natural conditions are favorable and continue.
Cline The establishment of plant populations over a specific geographic range that have adapted to different locations and have become slightly different from one another. The plants show a gradient of change over the range, with the frequency of a particular gene either increasing or decreasing over the range. Under the right conditions, speciation may occur over time.
Cloaca An all-purpose opening that serves as a digestive, excretory, and reproductive tract for most vertebrates, with the exception of the majority of mammals.
Clonal deletion A mechanism whereby the loss of lymphocytes of a particular specificity is due to contact with either "self" or an artificially introduced antigen.
The faces of identical "Megan" and "Morag," the world’s first cloned sheep aged 9 months. These Welsh mountain sheep were the product of research by Dr. Ian Wilmut and colleagues at the Roslin Institute in Edinburgh, Scotland. The research involved culturing identical embryonic cells from sheep to produce a "cell line." Next, a sheep egg cell had its DNA removed, and one of these embryonic cells was implanted into the egg. A spark of electricity then stimulated the egg to grow into a lamb, nourished in the womb of a surrogate sheep. The ability to clone farm animals, first achieved in 1996, may provide benefits to agriculture and biotechnology.
Clonal selection theory (Burnett theory) Clonal selection theory states that the specificity and diversity of an immune response are the result of selection by an antigen of specifically reactive clones from a large repertoire of preformed lymphocytes, each with individual specificities.
Clone A population of organisms, cells, viruses, or DNA molecules that is derived from the replication of a single genetic progenitor. In the case of B cells, each B cell has a typical Ig, and so all the cells that descend from one B cell (the clone) have the same Ig. Typically, a cancer is a clone of cells. Sometimes, clone is also used to refer to a number of recombi-nant DNA molecules all carrying the same inserted sequence.
Cloning vector Any organism or agent (virus, plas-mid) that is used to introduce foreign DNA into host cells.
Closed circulatory system A type of circulatory system where the blood is contained within a system of vessels and the heart; blood vessels carry blood through all the organs.
Cluster A number of metal centers grouped closely together that can have direct metal-bonding interactions or interactions through a bridging ligand, but are not necessarily held together by these interactions. Examples can be found under the entries [2fe-2s], [4fe-4s], ferredoxin, hipip, iron-sulfur cluster,femo-cofactor, ferritin, metallothionein, nitro-genase, and rieske iron-sulfur protein.
Clusters of differentiation (CD) Cluster of antigens, with which antibodies react, that characterize a cell surface marker.
Lymphocytes can be divided into subsets either by their functions or by surface markers. The availability of monoclonal Abs raised against lymphocytes has allowed for the demonstration of several lymphocyte subsets, which express a combination of certain molecules on their surfaces. These surface markers have been designated clusters of differentiation (CD). Already, 78 CDs have been identified as well as the monoclonal Abs used to define them, their molecular weights, and cellular distribution. For example, CD23, the receptor for the FC portion of IgE and CD8 (T8), a protein embedded in the cell surface of suppressor T lymphocytes.
CD4 One of the most infamous CDs due to its importance in AIDS. CD4 (T4) is the protein embedded on the surface of T helper and other white blood cells to which HIV attaches itself. It is also found to a lesser degree on the surface of monocytes/ macrophages, Langerhans cells, astrocytes, ker-atinocytes, and glial cells. HIV invades cells by attaching itself to the CD4 molecule (CD4 receptor). The number of T4 cells in a blood sample is used to measure the health of the immune system in people with HIV.
A closed circulatory system, where the blood is contained within a system of vessels and the heart.
Helper T cell (CD4 cell, helper, helper cell, T helper cell, T helper lymphocyte, T4 cell) A subset of T cells that carry the T4 marker and are essential for turning on antibody production, activating cyto-toxic T cells, and initiating other immune responses. The number of T4 cells in a blood sample is used to measure the health of the immune system in people with HIV. T helper lymphocytes contain two subsets, TH1 and TH2 cells.
CD8 cell (T suppressor cell, T8 cell) The existence of these cells is a relatively recent discovery, and hence their functioning is still somewhat debated. The basic concept of suppressor T cells is a cell type that specifically suppresses the action of other cells in the immune system, notably B cells and T cells, thereby preventing the establishment of an immune response. How this is done is not known with certainty, but it seems that certain specific antigens can stimulate the activation of the suppressor T cells. Discrete epitopes have been found that display suppressor activity on killer T cells, T helper cells, and B cells. This suppressor effect is thought to be mediated by some inhibitory factor secreted by suppressor T cells. It is not any of the known lymphokines. A fact that renders the study of this cell type difficult is the lack of a specific surface marker. Most suppressor T cells are CD8 positive, as are cytotoxic T cells.
Clutch The eggs laid in a nest by an individual bird.
Cobalamin (vitamin B12) A vitamin synthesized by microorganisms and conserved in animals in the liver. Deficiency of vitamin B12 leads to pernicious anemia. Cobalamin is a substituted corrin-Co(III) complex in which the cobalt atom is bound to the four nitrogen atoms of the corrin ring, an axial group R and 5,6-dimethylbenzimidazole. The latter is linked to the cobalt by the N-3 nitrogen atom and is bound to the C-1 carbon of a ribose molecule by the N-1 nitrogen atom. Various forms of the vitamin are known with different R groups, such as R=CN, cyanocobalamin;R=0H, hydroxocobalamin; R=CH3, methylcobalamin; R=adenosyl, coenzyme B12.
Sound signals pass from the cochlea via the oscillating hair cells, which transform them into electrical signals along the auditory nerve to the brain stem, where they activate other nerves in the brain.
Cochlea The inner ear; a circular or coiled snaillike shell that contains a system of liquid-filled tubes with tiny hair cells. Sound signals pass from the cochlea via the oscillating hair cells, which transform them into electrical signals along the auditory nerve to the brain stem, where they activate other nerves in the brain.
Cockroach The order Blattodea that contains the insects also known as the "cucaracha," black beetle, water bug, Yankee settler, shiner, and a host of other names; it is one of the most hated insects known to man. There are 4,000 species, but only about 12 are commonly associated with humans. Common species include: Blatella germanica, Blatta orientalis, Periplane-ta americana, Periplaneta australasiae, Periplaneta brunnea, Periplaneta fuliginosa, and Supella longipalpa.
Codominance When both alleles in a heterozygote are expressed phenotypically.
Codon A sequence of three consecutive nucleotides that occurs in mRNA and (a) directs the incorporation of a specific amino acid into a protein, or (b) represents the starting or termination signal of protein synthesis.
Scanning electron micrograph (SEM) of a vertical section through part of the cochlea inside a human ear. The section shows part of the row of columnar outer pillar cells that runs along the organ of Corti, the auditory sense organ. The outer pillar cells arise from the basilar membrane (across bottom), and their upper surfaces (across top) form part of the surface of the organ of Corti. This organ lies on the basilar membrane, an internal surface of the cochlear duct. The organ of Corti also contains hairlike cilia (not seen) and an overlying tectorial membrane (removed). Sound waves deform hairlike cilia and trigger auditory nerve impulses. Magnification: x600 at 6 x 7 cm size.
A codon consists of three bases in a DNA or RNA sequence that specify a single amino acid.
Coefficient of kinship The kinship coefficient expresses the chance of finding common genes on the same locus. It also expresses the probability that alleles drawn randomly from each of two individuals are identical by descent. It is also the relationship between a pair of individuals.
Coefficient of variation A measure of dispersion around the mean (average).
