chromosome

chromosome , structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome is usually reserved for the structure when it is condensed and readily visible during cell division (see mitosis ). At other times the chromosome appears as a fibrous structure, called the chromonema, consisting of accumulations (called chromomeres) of chromatin, the dye-absorbing material. During nuclear division, when each chromosome splits, each of the duplicate chromosomes is called a chromatid. A certain number of chromosomes is characteristic of each species of plant and animal; e.g., the human has 46 chromosomes, the potato has 48, and the fruit fly Drosophila has 8. Each of these chromosome numbers is the so-called diploid number, i.e., the number found in the somatic (body) cells and in the germ cells that give rise to the gametes, or reproductive cells. When the germ cells divide in the two-step process of meiosis , the chromosomes are separated in such a way that each daughter cell receives a haploid (half the diploid) number of chromosomes. Fusion of the male and female gametes in fertilization restores the diploid number in the fertilized egg, or zygote, which thus contains two sets of homologous chromosomes, one from each parent. The principal constituents of the chromosomes are nucleoproteins containing deoxyribonucleic acid, or DNA (see nucleic acid ). Chromosomes appear microscopically as a linear arrangement of genes, the factors that determine the inherited characteristics of all living organisms. The very large chromosomes in the salivary gland cells of Drosophila and other insects have furnished valuable material for the study of genetics .

chromosome A protein thread composed of DNA and histone, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA: these are not chromosomes, though they serve a similar function. Chromosomes occur in pairs. Each species tends to have a characteristic number of pairs of chromosomes (23 in humans), found in most nucleated cells within most organisms. The presence of pairs of homologous chromosomes is referred to as the diploid state and is normal for the sexual phase of an organism. Gametes (reproductive cells), and cells of the gametophyte (gamete-producing phase) of plants, however, have only one member of each pair in their nuclei (the haploid state).

chromosome A DNA-histone protein thread, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA: these are not chromosomes, though they serve a similar function. Chromosomes occur in pairs, which associate in a particular way during meiosis. Each species tends to have a characteristic number of chromosomes (e.g. 20 in maize, 23 in humans), found in most nucleated cells within most organisms. The presence of pairs of homologous chromosomes is referred to as the diploid state and is normal for the sexual phase of an organism. Gametes (reproductive cells), and cells of the gametophyte (gamete-producing phase) of plants (see alternation of generations), however, are haploid with only one member of each pair in their nuclei. Usually chromosomes are visible only during mitosis or meiosis when they contract to form short thick rods coiled into a spiral. Each chromosome possesses chromomeres and a centromere, and some contain a nucleolar organizer. Chromosomes contain a line of different genes, a spindle attachment at some point along their length, and regions of heterochromatin, which stains strongly with basic dyes.