MANNER OF PRODUCTION OF MUTATIONS 557 



found 2-A X rays as efficient as ordinary X rays in causing lethals in 

 Drosophila, they were inclined to attribute this apparent violation of the 

 target hypothesis to the production of mutations via the intermediation 

 of "activated water" (to use Fricke's term). It is to be noted, however, 

 that not all the X-ray mutations can be produced through reactions 

 originating in free water since even dried bacteria (Devi et at., 1947) and, 

 in the work of Stapleton and Hollaender (1952), fungus spores deprived 

 of both free water and oxygen still gave rise to some mutations as a result 

 of irradiation. 



Rapoport, who' was the most uncompromising speaker in defense of 

 genetics during the attack on this science in the U.S.S.R. in 1948, and 

 who has not been heard from since, was also the first investigator who, 

 generalizing from already existing results, came to the conclusion that 

 the mutagenic action of radiation must be exerted via the activated 

 oxygen-containing groups produced by it (Rapoport, 1943). He based 

 this conclusion in the first place on the similarity of both the chemical 

 and the biological effects of ionizing radiation and ultraviolet with those 

 of ozone, which is itself produced by ionizing radiation. His second 

 argument lay in the promoting influence on X-ray mutagenesis exerted by 

 iron (to a degree not explicable by its higher X-ray absorption) and by 

 some other substances which affect oxidations (see pp. 574-581). He was 

 inclined to regard the mutagenic effect of radiation as brought about 

 largely by the ozonidation, by ozone itself, of unsaturated bonds in the 

 genetic material, but he also called attention to the manifold effects on 

 organic molecules of the substances of high oxidizing potential which are 

 produced by radiation. He further pointed out that even spontaneous 

 mutations may arise in the same way, as a result of active oxygen-con- 

 taining groups, produced in this case in the course of metabolism. This 

 led him to various successful experiments in chemical mutagenesis (see 

 Chap. 7). Unfortunately Rapoport's papers seem not, until recently, to 

 have come to the attention of investigators outside the U.S.S.R., and it 

 was several years before they were able, through work of their own, to 

 come to similar conclusions. At the same time, the details of Rapoport's 

 interpretations, such as the conception that ozone itself serves as the 

 chief intermediary substance, that it attacks the genetic material directly, 

 and that unsaturated bonds are the main sites of its attack, remain 

 highly questionable. 



The promotion of the production of point mutations and chromosome 

 breaks by the presence of oxygen during exposure to ionizing radiation is 

 now well established. Parallel with the work of Stone, Wyss, and 

 co-workers who. starting in 1947, showed the important role played in 

 short-wave ultraviolet mutagenesis by chemical groups containing a 

 highly reactive oxygen atom, Thoday and Read (1947) showed that the 

 frequency of chromosome changes as well as the amount of damage to 



