Natural and Induced Chromosomal Changes 



179 



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FIGURE 21-5. The relation between X-ray dosage and the frequency of breaks 

 induced in grasshopper chromosomes. {Courtesy of J. G. Carlson, Proc. Nat. 

 Acad Sci., U.S., 27:46, 1941.) 



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10 20 30 40 50 60 70 80 90 100 110 120 130 

 DOSAGE IN R UNITS 



paragraph, it should be no surprise to you 

 that the number of breaks produced by the 

 direct action of a given dose of a particular 

 radiation will depend upon the volume which 

 a chromosome occupies; this volume may 

 be different at different times in the nuclear 

 cycle, and the same chromosome may occupy 

 different volumes in different tissues or sexes. 

 It is also reasonable, in view of the fact that 

 breakage is energy-requiring, that the num- 

 ber of breaks produced indirectly is increased, 

 if during irradiation either the amount of 

 oxygen is increased or the cell's reducing 

 substances are poisoned. As expected, there- 

 fore, replacement of oxygen by nitrogen dur- 

 ing irradiation reduces the number of breaks 

 produced. 



With this preliminary discussion of some of 

 the factors influencing the production of ra- 

 diation-induced breaks as a background, con- 

 sider next the factors influencing the fate of 

 the ends produced by breakage. Just as 

 breakage involves a chemical reaction, so 

 does the union between two broken ends. It 

 has been shown that the joining of broken 

 ends also requires energy, being enhanced by 

 the presence of oxygen (and prevented by 

 nitrogen) when present after irradiation. So, 



restitution is prevented if nitrogen has re- 

 placed oxygen after irradiation. This in- 

 creases the time that the ends from the same 

 break stay open, and this often increases their 

 chance for cross-union later in the presence 

 of oxygen. (You see, therefore, that oxygen 

 has two contrary effects on rearrangements, 

 its presence during irradiation increasing the 

 number of breaks and its presence after 

 irradiation increasing restitution.) 



Since, under a given set of conditions, the 

 number of breaks increases linearly with dose, 

 each part of the dose independently produc- 

 ing its proportional number of breaks, it is 

 clear that the number of breaks produced is 

 independent of the rate at which a given total 

 dose of radiation is administered. It also 

 follows, then, that all structural changes 

 consequent to single breakages are also inde- 

 pendent of the radiation dose rate. Radia- 

 tions, like fast neutrons, which produce tracks 

 that are long and dense with ionizations, may 

 frequently produce two breaks with the same 

 track. In this case, if the same chromosome 

 is broken twice because, having coiled tightly, 

 it lay in the path of the track twice, large and 

 small structural changes of inversion, de- 

 ficiency, and duplication types may be pro- 



