Chromosome Studies on Trillium kamtschaticum Pall

XV. A contribution to the present status of knowledge on the mechanism of chromonema coiling

抄録

In the present paper the configuration assumed by the chromonema in chiasma loops and chromatid bridges was studied, with an aim to ascertain how the chromonema coiling in such arms both ends of which are fixed differs from the previous findings on free arms whose distal end is free and to touch in this way of comparison the problem of the mechanism of spiralisation. Observations on these “fixed” strands have shown that:-
1) When they are below a certain limit of length, they are not able to assume regular cylindrical spirals, but take merely wavy corrugated configurations. In “free” strands no such a conditions is met with.
2) Sometimes each of the paired chromatids is subjected to independent coiling. In free arms of the normal chromosomes such irregularity in coiling is never met with, the paired chromatids always forming a single fused spiral.
3) The spiral configurations assumed by these, when they are relatively long, are characterized by a profound increase in frequency of reversals in coiling direction as contrasted with the case of free strands previously studied.
4) In these strands the spiral of the balanced type (Fig. la) occurs predominantly; especially in the true terminal chiasma loops, the configurations are always of this type.
From these findings and other sources of evidence previously presented it has been inferred that:-
1) Pairing of the homologues takes place after the leptotene threads completed the untwisting of the relic twists; they conjugate thereafter side-by-side in one, limited pairing surface of each; the four strands which are to separate two-by-two at diplotene lie thus essentially parallel.
2) “Free” strands are able to rotate in spiralisation and consequently the parallel arrangement of the paired chromatids can be converted into the twisted orientation in spirals, that is, the relational spiral, while in “fixed” strands their freedom of rotation is prevented and therefore the paired chromatids take more complicated configurations as the present paper has shown. In the latter, the paired strands will assume either a relational spiral in the form of a balanced spiral or a parallel one in the form of an unbalanced spiral.
3) For what determines the pitch and the regularity of the spiral in metaphase chromosomes the following three factors are considerable: (a) the space delimited by the matrix, (b) the elasticity of the chromonema and (c) the electrical charges to be accepted by the chromonema at metaphase plate which determine at the same time the external chromosome mechanics. These are responsible for the formation of the major as well as the minor spiral.