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Chromosome, threadlike body in the cell nucleus, composed of genes, which carry genetic information responsible for the inherited characteristic of all organisms. Chromosomes consist of deoxyribonucleic acid (DNA), a sequence of nucleotides composed of 4 different bases, allowing over 500 million alternatives. The basic proteins are found in a complex with DNA in the cells of human organs and tissues; in general, these are called histones. The microscope shows that the chromosome is a coiled structure. This coiling should not be confused with the structure of the DNA molecule—a double helix— because there is a difference of several orders of magnitude in the scale. Mitosis is the normal process by which a cell divides, each new cell ending up with the same number of chromosomes as the original cell. Mitotic division occurs in somatic cells (i.e., not sex cells) during periods of growth, when the total number of cells is increasing, or during repair processes, when lost or damaged cells are being replaced. In the resting cell (interphase of mitosis) the chromosomes cannot be detected inside the nucleus as discrete structures. As a cell approaches mitosis, the chromosomes become visible as threads within the nucleus (prophase of mitosis). At the same time, cytoplasmic bodies, the centrioles, divide, and the 2 new centrioles move to opposite poles of the cell. The protein fibers that control the separation of the chromosomes radiate from these bodies. The chromosomes line up in the equatorial plane of the cell (metaphase of mitosis) and separate to form the chromosomes of the 2 new nuclei (anaphase of mitosis). A cleavage furrow begins to divide the cytoplasm of the cell, the chromosomes elongate, and a nuclear membrane re-forms around them. This stage (telophase of mitosis) results in the reconstruction of 2 nuclei that are genetically identical with the parent nucleus.

See also: Genetics; Heredity; Deoxyribonucleic Acid.

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