Structural Changes in Chromosomes 



165 



trie one is pulled toward both poles at once. 

 The acentric isochromosome is, therefore, 

 not included in either daughter nucleus and 

 so is lost to both. (The acentric pieces in 

 diagram 3 will be lost in any subsequent 

 division, whether they are joined to each 

 other or do not join at all.) The dicentric 

 isochromosome. being pulled to both poles 

 at once, forms a bridge. A bridge can pre- 

 vent any part of the chromosome from enter- 

 ing either daughter nucleus, so that the 

 dicentric is lost. Alternatively, the centric 

 regions of the dicentric piece can enter the 

 daughter nuclei, and the bridge can either 

 snap at any one of a number of places be- 

 tween the centromeres and free the daugh- 

 ter nuclei from each other, or it can persist, 

 joining the daughter nuclei together. 



The amount of phenotypic detriment that 

 a single nonrestituting chromosome break 

 will produce in the daughter cells and their 

 progeny cells depends upon the particular 

 chromosome involved, the place of breakage, 

 and the fate of the dicentric piece. Suppose. 

 for example, that chromosome IV of Dro- 

 sophila (often viable as a haplo-IV indi- 

 vidual) is the chromosome involved. The 

 break can occur at any position in IV, and 

 the loss of the genes in the acentric piece, 

 though detrimental, does not usually cause 



death; neither does the loss of the entire 

 dicentric fragment if excluded from both 

 daughter nuclei, nor, probably, does a snap 

 in the bridge between the daughter nuclei. 

 (In the last case, each daughter nucleus is 

 deficient, at least for the genes in the acen- 

 tric piece.) 



Note what happens when a bridge, involv- 

 ing a dicentric isochromosome linearly dif- 

 ferentiated as a.bcddcb.a (the centromere 

 is between a and b). does not snap between 

 the d's. If it breaks between b and c, one 

 fragment is even more deficient (yet viable 

 in this example), whereas the other con- 

 tains an extra dose of the genes in the cd 

 region (and is most probably viable). Re- 

 gardless of where the bridge snaps, both 

 daughter nuclei carry a centric fragment 

 which, after replicating, usually forms a new 

 dicentric isochromosome and can again form 

 a bridge at the next mitotic division. It is 

 possible, therefore, to have bridge-breakage- 

 fusion-bridge cycles in successive nuclear 

 generations. 



When a bridge fails to break leaving the 

 two daughter nuclei tied together, the en- 

 tanglement of the nuclei may interfere with 

 subsequent attempts at nuclear division. In 

 our example, this interference may be of 

 much greater importance than the presence 



figure 12-1. Conse- 

 quences of a single 

 nonrestituting 

 chromosome break. 



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