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AN INTRODUCTION TO MODERN GENETICS 



"super-Spirals" which disappear as the chromosomes shorten). During 

 the unwinding of the relic spirals, the chromosomes are already con- 

 tracting and assuming the spiral which will be fully developed at the 

 new metaphase. The remarkable fact emerges that these new spirals 

 are not developed directly from the reUc coils, but arise quite inde- 

 pendently of them. Darlington states that the new coils always have a 

 smaller amplitude than the reUc coils, and gives it as a general rule that 

 spiralization always proceeds from numerous coils with a small diameter 

 of each turn to less numerous coils with a larger diameter. 



The second phenomenon is that of relational coiling. When two 



Fig. 148. Relational Coiling. — Three bWalents of Chorthippus in pachytene or 

 early diplotene. Note the relational coiling of the chromatids and of the chromo- 

 somes. Each bivalent contains three chiasmata, indicated by dots (cf. Fig. 65). 



(From Darlington.) 



chromosomes pair in zygotene, they become coUed round one another; 

 and when each chromosome spUts in pachytene, it is seen that the sister 

 chromatids are coiled round each other. This can be explained if we 

 assume that each of the paired chromosomes is attempting to uncoil 

 its reHc coils, but that it is so closely associated with its partner that it 

 is unable to rotate around its axis. The mechanism can very easily be 

 demonstrated with two thick strands of wool. Twist each strand in the 

 direction in which it has already been twisted during spinning, place 

 the two side by side and let go. Each strand will now attempt to un- 

 twist; and this can be taken as analogous to the attempt to uncoil which 

 we have assumed for the chromosomes. But it will be found that the 

 small hairs on the two strands of wool become entangled and prevent 

 the strands slipping over each other; the result will be that the strands 

 become partially untwisted and also coiled round one another in a 

 "relational spiral." 



