STRUCTURAL CHANGE 



self-propagating organ like any other gene or group of genes, 

 concerned with the organization of proteins on the spindle just as 

 others are in the nucleus. 



Centric fragments on the other hand, move normally on the 

 spindle. They are distributed to the daughter nuclei and appear in 



hi 

 Split 



r 





<■-- 



2.b" 



— > 



II (T^ f^ 



Split 



SR 



Post 

 Split 



K 



<--- 



-> 



Fig. 20. — The chief successions of change that may follow two spontaneous or 

 induced breaks (B") in two chromosomes with restitution, chromosome reunion 

 (R"), sister reunion (SR) and chromatid reunion (R') — in this order — at later 

 stages. The pre-split stage is represented with the chromosomes divided for ease of 

 comparison. Each step may be fmal and hence observable at the ensuing metaphase. 

 SR is independent on the centric and acentric sides of a break (from Darlington 

 and KoUer, 1947). 



inheritance as chromosomes lacking particular blocks of genes. If 

 too many genes are missing from a fragment the unbalance from 

 this deficiency may be fatal even where its homologue is intact. 

 Indeed X-ray experiments show that in diploid organisms most cells 

 whose chromosomes are deficient, owing to breakage, never divide 

 again after the mitosis where the fragment is lost. 



A single break can lead to nothing but loss; two breaks begin 

 to show us more fruitful possibilities. The broken ends can rejoin 



lOI 



