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, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
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chromosome substance found in the nuclei of plant and animal cells. Chromatin stains intensively with nuclear stains and, at the time of cell division, forms certain visible structures in the chromosomes. The term was introduced in 1880 by the German histologist W. Flemming. Present-day cytologists generally understand chromatin to be chromosomal material of the cell nucleus in interphase (between its successive divisions), since chromosomes in that period of cell cycle are not easily detected under the microscope. Chromatin is made up of deoxyribonucleic acid (DNA, 30–40 percent), ribonucleic acid (RNA), histones, and nonhistone proteins. The main structural components of chromatin are deoxyribonucleoprotein strands measuring 100–200Å in diameter and based on, according to most investigators, one molecule of DNA.
American scientists have proposed two models of the fine structure of a primary chromatin strand: super-coil (J. F. Pardon and M. H. F. Wilkins, 1972) and spheroid (R. D. Kornberg, 1974; A. L. Olins and D. E. Olins, 1974). The spheroid model, which has been better substantiated experimentally, supposes that the primary chromatin strand is a flexible chain of repeating subunits—that is, nucleosoma—which is a bent DNA section of 150–200 pairs of nucleotides and a complex of eight histone molecules.
Genetically active chromatin (euchromatin) is differentiated from inactive chromatin (heterochromatin). The cell nuclei of females of many organisms—especially mammals (including man)—contain dense masses of chromatin called sex chromatin. Such masses, which are not present in males, apparently are formed in females by inactive sections of the sex chromosomes, mainly from the heterochromatin of one of the paired X chromosomes.
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