396 



Intrachromosomal Aberrations 



and fails to be included in the daughter nuclei. Sometimes, how- 

 ever, the ring breaks and the centromeres can move to the poles. 

 Each daughter nucleus will then have a segment of this original 

 ring. The two broken ends of each ring unite, and thus new rings 

 are formed. Since the place where the dicentric ring may break 

 is not necessarily the place where the two original sister halves 



Anaphase 



7 



b 



4 



b 



2 



8Kj 



Late anaphase Telophase Late anaphase 



/^^ 



K:^ 



r^ 



:o: 



Late anaphase Telophase 

 1 8 



r\ 



^' 



Fig. 103. McClintock's explanation of ring chromosomes in maize. A 

 ring chromosome is shown in the resting stage at upper left; the small 

 oval is the centromere and the numbers represent segments of the chromo- 

 some. At upper middle is a prophase showing a "crossover" between the 

 two sister chromatids of the divided ring chromosome. When this is 

 oriented on the spindle a dicentric is seen as at upper light. The chromatin 

 strands break at various places. If a break occurs at a-a, new ring chro- 

 mosomes will form as at lower left; if the break is at b-b, the new ring chro- 

 mosomes will be of unequal size as at lower middle; a break at c-c will 

 produce the rings at lower right. (Courtesy of Dr. B. McChntock in 

 Genetics.) 



