366 RADIATION BIOLOGY 



Sertoli) cells (Miiller and Settles, 1927). However, as has been stated 

 above, the defect will usually — unless the missing chromatin is exception- 

 ally small or unimportant — cause abnormalities sufficient to kill the 

 individual of the next generation at an early stage of its development. 

 This occurs despite the fact that its cells, being diploid, contain one 

 normal set of chromosomes. 



Convincing evidence has been reported that in some material, for 

 example, maize sporophytes (McClintock, 1939), a chromosome broken 

 by ionizing radiation which fails to make contact mth another broken 

 end may after a time undergo healing, in that the broken end permanently 

 loses its adhesive property so as to be able to function like a normal 

 unbroken chromosome end. If this occurred, the centric fragment could 

 reproduce itself without danger of forming a dicentric isochromosome and 

 so becoming lost or killing the cell. It would therefore be carried along 

 in mitosis like any other chromosome. But the cells with this chromo- 

 some would nevertheless be deficient for the acentric fragment, having 

 what is called a terminal deficiency of the affected chromosome. They 

 would therefore be genetically abnormal, and this deficiency would still 

 be enough, in the vast majority of cases, to kill the gametophytes. 

 Ultraviolet fight has been reported by Stadler (1939) and by Swanson 

 (1942) to be especially conducive, in some plant material, to producing 

 this effect. In animals, on the other hand (and also in maize gameto- 

 phytes and endosperm when the breakage is mechanical or by ionizing 

 radiation), the evidence is all against the occurrence of healing, despite 

 some contrary claims. It may be concluded that, at least in those 

 animals studied, the free unbroken ends of each chromosome have a 

 characteristic structure, which a broken end cannot ordinarily assume, 

 and that this structure is necessary for the continuance and orderly 

 distribution of the chromosome through repeated mitoses. This prop- 

 erty justifies us in distinguishing the normal free ends in such organisms 

 as telomeres, in contrast to the interstitial portions of the chromosome, 

 which include the centromeres. 



5. CONSEQUENCES OF TWO BREAKS IN SEPARATE CHROMOSOMES 



When two chromosome breaks have occurred in the same nucleus, each 

 of the breaks may be followed by one of the types of behavior pattern 

 already described. However, the alternative possibility now arises of 

 one broken end meeting and forming a union with an end derived from a 

 different break. The type of rearrangement which thereupon results 

 depends on where in the chromatin the breaks are located and which 

 broken ends unite with which. 



If, as is more often the case, the two breaks in question occur in different, 

 nonhomologous chromosomes, and a fragment of one of these chromo- 



