MUTATION AS A CHEMICAL PROCESS 



223 



intermittent, there is a greater chance that the break will heal in the 

 original configuration before a second break occurs that would allow an 

 aberration. X-ray-induced chromosomal aberrations are observed more 

 frequently at high oxygen tensions, which has led to the belief that 

 X-rays act in an indirect way. Oxygen is important in the formation of 

 H2O2 and HO2 in irradiated water, and it may be these products which 

 induce the breaks. Yet it is still possible to contend that oxygen in- 

 fluences the way in which breaks rejoin; probably it acts in both ways. 

 Many chemicals will also induce chromosome breaks but ultraviolet light 

 induces relatively few. As a matter of fact, UV-induced mutations re- 

 semble spontaneous mutations more than do those induced by X-rays. 



Ultraviolet radiation is not ionizing; it consists of quanta with less 

 energy and longer wave lengths than X-rays. Substances such as nucleic 



0.8 - 



0.6- 



0.4 



^ 0.2 



90 



180 270 



X-ray Dose (r) 



360 



450 



FIGURE 8.12. Relation of chromosomal aberrations in the microspores of Tradescantia, 

 to X-ray dosage (from Giles and Riley, 1949, Proc. Natl. Acad. Sci. Wash., 35:640). 



