Meiosis 57 



When one chromatid breaks at a certain place, one of the 

 two chromatids of the other chromosome also breaks at exactly 

 the same place. The broken ends then uncoil. The broken end 

 of one chromatid in some way seems to come into contact with 

 the broken end of the other broken chromatid and they fuse. 

 This fusion largely eliminates relational coiling. If one such 

 break, followed by a fusion, occurs, one chromatid of each chro- 

 mosome remains unchanged but the other two chromatids are 

 new and are composed in part of one original chromatid and in 

 part of the other. Since the break is at the same place in each 

 of the two chromatids, the new chromatids are exactly the same 

 as the old ones in size and appearance ; but the new arrangement 

 of segments of chromatids has a very important effect on the 

 transmission of groups of genes, resulting in what is known 

 genetically as crossing over. Since more than one break-and- 

 fusion usually occurs in a pair of homologous chromosomes, and 

 since they may involve any chromatid of either pair at any one 

 place, the results in terms of the original nature of the four 

 threads can be quite complicated. 



The breaks and exchanges of partners produce cross-shaped 

 figures in the paired chromosomes when viewed under the micro- 

 scope. They are best observed in the next stage of the first 

 meiotic prophase, diplotene. During diplotene, the repulsion be- 

 tween the pairs of chromatids is very strong, and the two pairs 

 now tend to separate from one another. They cannot do so 

 completely, however, because at various places one chromatid 

 from one chromosome is attached to a piece of one chromatid 

 from the other chromosome. The parts that are not joined 

 separate as widely as they can. If one break had occurred in 

 pachytene, the two homologous chromosomes would present a 

 cross-shaped figure, and the length of the arms of the cross would 

 depend upon the original position of the break. If more than 

 one break had occurred, the chromosomes would open out into 

 loops. The regions where they are tied together as the result 

 of the previous breaks are known as chiasmata. 



The two homologous chromosomes which have paired at zygo- 

 tene are known as a bivalent. When four threads are present as 

 the result of the doubling of each chromosome, the configuration 

 is known as a tetrad. The tetrad nature of a bivalent is not 



