The internal mechanics of the chromosomes III—relational coiling and crossing-over in Fritillaria

Abstract

I have now outlined some of the considerations on which a study of this problem must rest. I have tried to show that the cyclical formation and resolution of spiral structure in the chromosomes at each mitosis entails a series of unforeseen changes which affect every aspect of chromosome behaviour. While the length of the chromosome regularly alternates between a maximum in the resting stage and a minimum at metaphase or anaphase, the degree of coiling of the chromosomes passes through a cycle of changes which lags behind this more elementary cycle. The complex conditions resulting from this lack of co-ordination can be explained by assuming that the changes in length are determined by the changes in internal structure which I have referred provisionally to the formation of a “molecular spiral.” The uncoiling of the spirals formed in each mitosis is then delayed owing to a lag in the mutual adjustment of the chromosomes in the crowded telophase and prophase nucleus. The chromatids formed by the division of chromosomes in the resting stage are therefore coiled round one another in such a way as to compensate for the relic coiling of the chromosomes in which they jointly partake. What kind of behaviour are we therefore to expect at meiosis ? In this exceptional kind of division, prophase begins precociously, before the chromosomes divide, and this is a condition of their pairing (Darlington, 1932, and Part II). It also begins, according to the only available observations, before the maximum extension of the chromosomes is reached. In Lilium at the end of pachytene the chromosomes are about one and a-half times their length at an earlier stage of prophase (Belling, 1931). A similar increase is found in the marsupials, Dasyurus and Sarcophilus (Koller, 1935).