384 RADIATION BIOLOGY 



touch and join together, giving an acentric and a dicentric isochromosome, 

 respectively. The dicentric chromosome, pulled to both opposite poles 

 at once, can then be seen (as in Fig. 7-le) to form a chromatin bridge 

 between the daughter groups of chromosomes at the next anaphase. 

 Thus irradiation during interphase gives relatively few structural changes, 

 and chief among these are chromatin bridges and their complements, 

 lagging acentric fragments, while such types of aberrations as trans- 

 locations, large deletions, and large inversions are rarely to be found. 

 And, as later and later interphase stages are irradiated, lying nearer and 

 nearer in time to the stage of chromatid formation, these bridges and 

 fragments are produced in ever greater abundance, since less and less 

 time is afforded for restitutional contacts to occur before the homologous 

 broken ends of the adjacent twin pieces become available for union with 

 each other. 



If, now, irradiation is carried out during some stage of mitosis, other 

 factors come into operation which hinder restitution and favor the 

 eventual union of ends derived from different breaks, resulting in struc- 

 tural changes. It has long been known that when chromosomes are 

 irradiated while they are in the tightly spiralized, condensed condition 

 characteristic of late prophase, metaphase, anaphase, and early telophase, 

 they do not in most cases appear to be broken at the time, although there 

 are exceptions in some material [see, for instance, A. R. Whiting's (1945) 

 results on meiotic divisions of Habrobracon]. However, when the con- 

 densing chromosomes reappear at the next mitosis, after the intervening 

 interphase has elapsed, structural changes of varied types are to be found 

 among them in relatively great abundance, as compared with what would 

 have followed irradiation during most of the interphase period. It is evi- 

 dent from this result that the chromosomes when in a condensed condition 

 are in most cases not able to fall apart into fragments, because of some 

 enveloping material, but that their inner threads nevertheless become 

 effectively broken by radiation. Moreover, while in this condensed 

 stage, the broken ends of the threads, although bound close together 

 passively by the material which prevents the pieces from falling apart, 

 are for some chemical reason unable to adhere actively to one another so 

 as to undergo actual restitution, for otherwise their potentiality of 

 later giving rise to strvicturally changed chromosomes would be lost. 

 It must therefore be concluded that the breaks persist, although invisible, 

 throughout the condensed stage, and that the pieces later, probably in 

 late telophase, tend to fall apart before the ends acquire their mutual 

 adhesiveness. As the chromosomes at about this time begin to enter 

 their relatively unspiralized, extended phase, their parts must undergo 

 much more movement relative to one another than before. Conse- 

 quently, when the broken ends have finally become adhesive, they are 

 now much less likely to find the other end from which they had been 



