The evolution of genetic systems / by C.D. Darlington.

  • Darlington, C. D. (Cyril Dean), 1903-1981.
Date:
1946
Catalogue details

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

Credit: The evolution of genetic systems / by C.D. Darlington. Source: Wellcome Collection.

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    8 * THE SUBSTANCE OF HEREDITY somes. Each centromere splits into halves, which move apart from one another towards the opposite poles, pulling their chro¬ matids or half-chromosomes after them. Between the two groups the spindle stretches, pushing them farther apart This stage of separation is anaphase. When the sets of chromatids come near the poles, two new nuclei are built up again. This last stage is telophase. While the nuclei are returning to their resting condition a fine spiral appears inside each chromosome, and its coils slighüy loosen and becoming closely entangled disappear in the optically homogeneous nucleus. The nuclear cycle is complete. If, how¬ ever, we compare this last view with the first we see that the coil s which gradually become fewer and straighten out during the contraction of prophase are a direct continuation of those seen at telophase. They are relic coils. And while they are disappearing at prophase we must suppose that a new spiral is developing inside each chromatid for the new mitosis. It is by developing an internal coil that the chromosomes con¬ tract lengthwise and become conveniently mobile during mitosis. When the coiling fails (exceptionally in certain Protozoa, and abnormally in higher organisms) the chromosomes remain long, so long that the daughter chromatids may fail to separate at anaphase, and a single nucleus is restored with a double number of chromosomes. Such under-coiled chromosomes have more coils than the normal but they are of smaller diameter, so it is clear that the whole nuclear cycle consists in a process of diminishing the number and increasing the diameter of coils in a chromosome. During prophase two successive coiling cycles overlap. Although the separation of chromatids takes place at anaphase it will be seen that the division of the chromosome into two chro¬ matids is accomplished during the previous resting stage, when the threads are dispersed in the nucleus and half uncoiled. How this division must be supposed to work is a molecular problem. It depends on the molecular structure we assume in the chromosome. It seems likely that the chromosome is a permanent carbon- nitrogen chain with long active side chains at intervals. Division would then consist in the attraction and laying down of identical component particles next to the original thread and these particles would fiise to make a new thread. It is an essential property of these essential materials of life that the attraction on which the division of the chromosomes depends