The Development of the Typical Forms of Gemini: The Origin of Crossing-over
Each of the eight gemini of
Clematis Jackmanii reaches during the phases of chromosome conjugation a typical form. Text-fig. I-1.
The typical forms of gemini are determined by the position and number of the first and second torsions.
Two chromosomes of the same pair lie in meiosis associated side by side by the reductional split, before they split into four chromatids (two and two daughter-chromatids).
The first torsion appears after the moment that the two chromosomes associated side by side split into two and two daughter-chromatids. This torsion is determined by the torsion at 180° round the axis of each of the two chromosomes of the same pair. This torsion is the same in meiosis as it is in mitosis. Models III-1.
The second torsion occurs before the moment that the two chromosomes associated side by side in a pair split into two and two daughter-chromatids: the two chromosomes turn one over the other. Models III-5.
At early diplotene the daughter-chromatids separate one from the other by their equational split until they reach the vertical position one to the other at the point of the first torsion: at this point an one side of the reductional split lie two daughter chromatids vertically one an the other, an the other side of the reductional split lie the other two daughter-chromatids in the saure way; before and behind this point the two dissimilar-chromatids remain associated side by side by the reductional split. This is the structure s.a. Text-fig. II-4: Models III-3, 14.
If there is also the second torsion in this geminus, when at diplotene the daughter-chromatids separate one from the other, then at its point the two dissimilar-chromatids associated side by side by the reductional split are turned over the other two dissimilar chromatids. (This is the structure s.b.) Models III-6, 10, 15-19, 21.
The composition of each of the eight gemini of the four ehromatids and the way of the intermingling of these four chromatids is best seen at highest diplotene. Models I.
The daughter-gemini separate at first anaphases one from the other by the reductional split; they then have a form similar to the forms of gemini in diakinesis. Text-fig. I-2, III.
If there are interchanges of corresponding Segments between the chromatids of a geminus during the phases of chromosome conjugation, these interchanges are accomplished before the geminus is at diakinesis.
The segmental composition of the four chromosomes resulting from the chromosome conjugation with only the first torsion remain unchanged. This corresponds to the gemini
f,
g,
h. Text-fig. III-1-3; Models III-4.
The chromosomes resulting from the chromosome conjugation with the first and the second torsion have interchanged corresponding segments. This corresponds to the gemini
a,
b,
c,
d,
e. Text-fig. III-4-8; Models III-9, 13.
The second torsion determines the origin of crossing-over: the crossing-over takes place between two dissimilar chromatids at the point given by the second torsion; the interchange of corresponding segments between two dissimilar chromatids is accomplished by the breakage and reunion before diakinesis. Text-fig. II-5.
If the second torsion is not fully at 180°, the crossing-over takes place between two dissimilar-chromatids: Each of the four chromosomes resulting from this chromosome conjugation has a different segmental composition; two have interchanged corresponding segments; the other two remain unehanged, each of them having the saure segmental composition as one of the two chromosomes of the same pair before their conjugation. This corresponds to the geminus
b. Models III-10-13.
If the second torsion is at 180°, the erossing-over takes place in both the two dissimilar chromatids. There is a difference in the position of these two points
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