chromosome aberrations in Tradescantia 755 



the cessation of radiation. The recent work of J. A. V. Butler and Con- 

 way (1950) indicates that this latter effect occurs only if oxygen is present 

 in the nucleic acid solutions at the time of irradiation. The immediate 

 depolymerization effect of X radiation is independent of oxygen con- 

 centration. It is noteworthy that the radiosensitivity of nucleic acid to X 

 rays increases by a factor of ~3 in the presence of oxygen over that found 

 under anaerobic conditions. This numerical increase is substantially the 

 same as that found for chromosomal aberrations induced in Vicia root tips 

 and Tradescantia microspores. Additional experiments are described 

 (ibid.) from which the conclusion is reached that H 2 2 is not the effective 

 agent when irradiation is performed in the presence of dissolved oxygen. 



In addition to the experiments on irradiation of nucleic acid in vitro, 

 others have been reported (Limperos and Mosher, 1950) on the effects of 

 irradiation in vivo. These workers conclude that substantial depoly- 

 merization occurs in living cells during irradiation, since they were able 

 to isolate almost completely depolymerized DNA from the thymus of 

 recently X-rayed rats. 



Although these experiments on the effects of X radiation on nucleic 

 acid are important in suggesting possible mechanisms of chromosome 

 breakage, they do not establish that depolymerization is necessarily 

 involved in this process. It is still not at all clear, for example, that 

 changes in the nucleic acid of chromosomes, as opposed to those in the 

 protein component, are the significant ones in chromosome breakage. 

 These experiments do indicate, however, that the structure of one impor- 

 tant component of chromosomes can be markedly modified by indirect 

 radiation effects mediated by radicals or related active substances. 



CONCLUSIONS 



In concluding this discussion of radiation-induced chromosome aber- 

 rations in Tradescantia, it appears desirable to consider to what extent 

 recent evidence, particularly that derived from the oxygen effect, necessi- 

 tates a modification of earlier conceptions of the mechanism of chromo- 

 some aberration production by ionizing radiations. Prior to the dis- 

 covery of the oxygen effect, the view was usually taken that the effect of 

 radiations on chromosomes was a direct one resulting from the ionization 

 of the bonding electrons of the molecules composing the chromosomes by 

 the passage of the ionizing particles (Catcheside and Lea, 1943; Lea, 

 1946). However, the demonstration of the marked effect of oxygen in 

 modifying the frequency of X-ray-induced aberrations, plus the evidence 

 that this effect is probably on the breakage and not on the recovery proc- 

 ess, appear to invalidate this opinion that direct molecular ionization is 

 the only mechanism involved. Rather, it now seems most probable that 

 the major fraction of the radiation effect on chromosomes is an indirect 

 one, resulting from the action of active radicals, or of related substances 



