THE CHROMOSOME MECHANISM 



fused to give the zygote we are examining. Thus, instead of the 

 diploid, 2M, number of chromosomes of mitosis, there is a reduced, 

 haploid, n, number of chromosomes. And instead of the paired 

 sister chromatids of mitosis produced by reproduction, there are 

 paired homologous chromosomes brought together by attraction. 



Suddenly attraction gives way to repulsion. All the pairs fall apart. 

 At the same time they are seen to be double. Each chromosome has 

 divided into a pair of cJiromatids. The chromosomes seem to remain 

 in contact at certain points and closer examination shows that their 

 chromatids exchange partners at these points. Apparently, under the 

 strain of coiling and at the moment of their origin by division, the 

 chromatids have broken, pairs of partner chromatids breaking at 

 corresponding points. They have uncoiled and rejoined in new com- 

 binations. This mechanical transformation is called crossing-over, the 

 observed change of partner is called a chiasma, and the stage is called 

 diplotene. 



Some of the coiling of the chromosomes is undone in the forma- 

 tion of chiasmata and some is lost afterwards by still more uncoiling. 

 The chiasmata can then be seen and recorded in number and position. 

 They are as a rule evenly distributed over those lengths of the 

 chromosomes which were paired at pachytene. When pairing has 

 been complete, their distribution is uniform throughout. When it 

 has been locaHzed, the chiasmata are of course similarly localized, 

 near the ends, or near the centromeres, or near both. The kind of 

 distribution of chiasmata is characteristic of each species or even 

 genus of plant or animal, e.g. complete and uniform in Lilium or 

 Zea Mays and pro-terminal in Triton, Tradescantia or Oenothera. 

 In Allium and Fritillaria there are some species with uniform, and 

 others with pro-centric, distribution. 



As uncoiling proceeds, the chromosomes thicken and shorten even 

 more than in mitosis. As this happens, adjoining loops between 

 chiasmata come to lie at right angles. The distal chiasmata (nearest 

 the ends) are pushed towards the ends of the chromosomes, which 

 then appear merely to touch. The compact paired, or bivalent, 

 chromosomes, as we may now call them, lie evenly in the nucleus. 

 Small chromosomes, where the centromeres are very close to the 

 nearest chiasma, are drawn into fme threads by the repulsion between 

 their centromeres. This stage is diakinesis. It is followed by the dis- 



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