The chromosomes / M.J.D. White.

  • Michael James Denham White
Date:
1973
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The chromosomes / M.J.D. White. Source: Wellcome Collection.

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    I The Interphase Nucleus We shall be concerned in this book with the physical basis of nuclear inheritance in higher organisms. In the bacteria, viruses and blue-green algae (the so-caUed prokaryota) bodies which have been called chromosomes exist and these have a clear analogy to the true chromosomes of the eukaryota. But the detailed mechanisms are substantially different. The broad distinction between the genetic systems of the prokaryota and eukaryota seems a fundamental one, although in recent years some evidence has been produced which suggests that the chromosomes of fungi (or at any rate some fungi) are biochemically simpler than those of the other eukaryota and in a sense intermediate between those prokaryota and eukaryota. All fungi, however, do seem to possess a true nucleus, bounded by a nuclear membrane, and these nuclei divide by mitosis. Thus the genetic systems of all the eukaryota seem to possess a fundamental unity which sug¬ gests that the fully evolved chromosomal mechanism, including mitosis, meiosis and fertilization, arose in an essentially mono- phyletic manner, in early Precambrian times probably over 2-5 X Iyears ago. Perhaps we might call the genetic systems of the viruses and bacteria prochromosomal and those of the eukaryota euchromosomal. If so, this book is about euchromo- somal mechanisms. It is, as yet, not possible to form any clear picture of the evolutionary origin of euchromosomal mechanisms from prochromosomal ones or of the intermediate stages which must have existed. Interphase, interkinetic or 'resting' nuclei are ones which are not undergoing the visible changes involved in cell-division. From a physiological standpoint we may distinguish between interphase nuclei which are between two mitotic cycles and those of 'terminal' cells which have entirely lost the capacity to undergo mitosis. Early embryonic tissues will contain only the former.
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