chromosome defined in 1951 yearchromosome - chromosome;
chromosome - Thread-shaped body, consisting largely of DNA and protein, numbers of which occur in the nucleus of every animal or plant cell. Bacteria and viruses have structures of similar function, but consisting of DNA (or in some viruses RNA) only. They occur in pairs, usually several different pairs per nucleus, in somatic cells of animals and higher plants (but See also: Endomitosis). The two members of each pair are identical in appearance and are said to be homologous (to each other). Homologous chromosomes associate in a characteristic way (pairing) during meiosis. Chromosomes of different pairs are often visibly different from each other in e.g. size, shape. There are one to over 100 pairs per nucleus according to species (man has twenty-three, Drosophila melanogaster four pairs) and most cells of most individuals of a given species have a set of similar chromosomes which is characteristic of that species. Gametes, and cells of the gametophyte of plants, have only one member of each pair in their nuclei. Chromosomes are usually visible only during mitosis or meiosis, when they contract in length, finally becoming short thick rods (a few micrometres long), probably by coiling up into a spiral. Fine threads have however been seen in the living resting nucleus which are probably the extended chromosomes. Chromosomes stain strongly with basic dyes (they are basophilic) during mitosis and meiosis.
Chromosomes are elaborately differentiated in structure along their length. This differentiation consists of a series of different genes in single file, presumably along a linear strand of DNA; a spindle-attachment at some point in their length; and regions of heterochromatin. The arrangement of these elements is practically the same in all homologous chromosomes. In some species the arrangement of many of the genes (i.e. of the loci) has been discovered (See also: Chromosome Map); it can be correlated with visible markings on the giant salivary gland chromosomes. A short time before every nuclear division (mitosis) each chromosome doubles and the two duplicates separate to the two daughter nuclei at mitosis. Essentially, this involves the replication of the DNA. For the behaviour of chromosomes during gamete formation, of great importance to genetics, See also: Meiosis; Sex Chromosome; Autosome; Salivary Gland Chromosome; Lampbrush Chromosome.
near chromosome in Knolik
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