364 RADIATION BIOLOGY 



to a daughter cell, and in this next cell generation, on reproducing 

 once more, its two chromatids will usually join to form a dicentric 

 isochromosome. 



When a cell containing chromosome fragments divides, any acentric 

 pieces, or acentric isochromosomes, lacking a spindle fiber attachment, 

 fail to become transported to either daughter nucleus. In consequence 

 the descendant cells are aneuploid, lacking this portion of one of their 

 chromosomes, and for this reason are genetically abnormal. If the 



(a) ■ 



(b) - 



(c) : 

 id) 



Fig. 7-1. Breakage of a chromosome prior to its splitting, followed by loss of the 

 pieces from the daughter nuclei, (a) The chromosome thread before breakage, with 

 the position of the centromere indicated, {h) The thread after breakage, composed 

 of an acentric and a cen/r/c fragment, (r) The two daughter threads, or "chromatids," 

 have become definitely established before the broken ends can unite, id) Broken 

 ends of twin fragments, being nearer to one another than to other broken ends, join 

 together, forming an acentric and a dicentric isochromosome, which are now becoming 

 more condensed, (e) In the ensuing cell division, the acentric isochromosome fails 

 to be pulled to the poles, and the dicentric one, pulled both ways, tends to form a 

 bridge. If this becomes broken by the tension, it repeats the process of dicentric 

 formation, and another bridge results at the next division, and so on, until the chromo- 

 some is lost or the cells are killed. 



missing part is large and important enough this deficiency can even cause 

 their death. As mentioned before in connection mth the loss of a whole 

 chromosome, such a defect in a germ cell would be especially likely to 

 cause the abnormality and the death of the individual derived from that 

 germ cell. 



As for the dicentric isochromosome that has been formed, its two 

 centromeres at the next mitosis are oriented toward opposite poles, just 

 as they would have been if the chromatids had not undergone breakage 

 and union, and the chromosome is thereby pulled in both opposite direc- 

 tions at once (see Fig. 7-le). Thus the dicentric isochromosome may 

 fail to enter one or both nuclei, and the resultant nucleus or nuclei, lack- 

 ing also the acentric portion, are rendered deficient for all of the broken 

 chromosome. Thereby the cells are caused to be more abnormal than 

 if they had lacked only the acentric part. Another complication is that 

 the dicentric isochromosome, in being pulled both ways, tends to form a 



