186 CHAPTER 13 



SUMMARY AND CONCLUSIONS 



The components ol structural chromosome change, breakage and cross-union, are 

 readilj studied through the use ol ionizing radiations. These radiations induce break- 

 aye in chromosome strands primarily by the clusters of ion pairs they produce. These 



clusters form tracks ol ions whose thickness and length determine the number and loca- 

 tion ol the breaks, hacks ol ions must occur very close to. or within, the chromosome 

 lh.it the) break. 



The number of breaks increases linearly with radiation dose. Whether they result 

 from one or from two breaks, all chromosomal rearrangements induced by a single 

 ionizing track increase linearly with radiation dose, have no threshold dose, and show 

 no effect from protracting or concentrating the dose. Accordingly, there is no dose 

 of ionizing radiation which does not produce breaks and at least single-track-induced 

 rearrangements. 



Two-or-more-break structural changes produced by ion clusters in separate, inde- 

 pendently-occurring tracks increase in frequency faster than the amount of dose and 

 do have a threshold dose. If joining of chromosome ends produced by breakage can 

 take place during the course of irradiation, such rearrangements are reduced in fre- 

 quency by protracting the delivery of the total dose. 



Since both the breakage and joining processes involve chemical changes, their fre- 

 quencies can be modified by the metabolic state of the cell. All types of rearrange- 

 ments are expected to be affected by: the physical and chemical state of the chromo- 

 some and the amount and distribution of its euchromatin and heterochromatin; by the 

 number and arrangement of the chromosomes present; by the presence or absence of a 

 nuclear membrane; and by the movements of broken ends as influenced by cellular 

 particles, fluids, and extracellular factors. 



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