Reference Entry


Michael Allaby

in A Dictionary of Geology and Earth Sciences

Fourth edition

Published in print January 2013 | ISBN: 9780199653065
Published online September 2013 | e-ISBN: 9780191744334

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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)....

Reference Entry.  130 words. 

Subjects: Earth Sciences and Geography

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