The genetics of micro-organisms / [D. G Catcheside].

  • David Guthrie Catcheside
Date:
[1951]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The genetics of micro-organisms / [D. G Catcheside]. Source: Wellcome Collection.

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    THE GENETICS OF MICRO-ORGANISMS multiple mating-type alleles (pages 33-5), numerical superscripts are the most convenient, e.g. mf-, mfi, etc. In those organisms, such as Basidio- mycetes (pages 101-3), in which there are two mating-type loci, the symbols mta^, mta^y etc., and mth^^ mtb^, etc., may be used. Where no ambiguity can arise the superscripts alone may be used for the mating- type alleles. Similar principles are applicable to other micro-organisms. Indeed, a uniform system has been used throughout this book in place of the diversity shown in the genetic literatures of the several organisms. Cytology In the fungi, the essential features of mitosis and meiosis (Fig. 2) are as in other plants and in animals. In the vegetative cells, the nuclei divide by mitosis so that each daughter nucleus is hke the mother one and contains a like number of similar chromosomes. During the resting stage prior to the division, each chromosome divides longitudinally into two chromatids (daughter chromosomes) except at one place, the centromere, where it remains undivided. One speaks of longitudinal division where most probably the basic event is the duplication of each chromosome in such a way that an exact reproduction is formed beside each. The chromosomes in the resting nucleus are slender extended threads, which become short compact rods during the prophase by a process of spiral packing. At the end of prophase, a spindle arises between two centrosomes and the centromeres of the chromosomes come to lie in its equatorial plane; this is metaphase. Then at anaphase each centromere divides and the daughter halves of each pass towards opposite poles of the spindle, taking with them their attached daughter chromosomes. A new nucleus is formed from each of the two polar groups of daughter chromosomes at telophase. In each there is a number of undivided chromosomes equal to the number of divided ones present at prophase in the parent nucleus. Neurospora crassa and other species have a chromosome number of seven. During meiosis, a reduction of the chromosome number from the diploid to the haploid number occurs, by two nuclear divisions accompanied by only a single division of the chromosomes. Unlike mitosis, the chromosomes enter upon the prophase of meiosis before they have divided. The two chromosomes of each homologous pair, recognized cytologically by their similar morphology, approach one another and come to lie closely side by side with like parts in close apposition, an association which is maintained throughout
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