The chromosomes / M.J.D. White.

  • Michael James Denham White
Date:
1973
Catalogue details

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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    20 THE CHROMOSOMES to be places where the two chromatids are held together closely. That this is in fact so can be seen if the cell has been exposed to a drug such as colchicine or colcemid; under these circum¬ stances the chromatids separate quite widely from one another, except at the centromere, which behaves as if it were undivided (whether it is actually unreplicated in a biochemical sense is doubtful). It is to be presumed that the DNA chromonema is continuous through the centromere. But it must be imspiralized so that the amount of DNA in the centromere region is too small to be revealed by ordinary staining techniques. In some organisms a small stainable chromomere can be seen in the middle of the unstained region, and some workers believe that this is the true organelle of attachment to the spindle. However, in many types of chromosomes no such chromomere is visible, so that it is probable that the unstained region should be regarded as the real centromere. In addition to the centromere, many chromosomes show other non-stained gaps or secondary constrictions, which are constant features of their structure; but these show no special relationship to the mitotic spindle. One or more of these secondary constric¬ tions may bear, during prophase, a bladder-like extension, the nucleolus, which usually shrinks and disappears before the begin¬ ning of the metaphase stage. The chromatids of the prophase chromosomes have an irregular, fuzzy outline and may appear as if covered with hairs or fine threads. By analogy with the giant lampbrush chromo¬ somes of amphibian oocytes (see Chapter 5) it is possible that these apparent threads are actually hundreds of looped structures. They are gradually withdrawn into the substance of the chromatid whose outlines become smooth just before metaphase. At the end of prophase the chromosomes have reached their maximum degree of condensation and contraction (Fig. 2). The nuclear membrane now undergoes disintegration and the spindle is formed, as a relatively stiff, gelatinous structure, in place of the original nuclear sap. The period when it is being organized is referred to as premetaphase; as soon as its forma-
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