OXYGEN EFFECT AND PREVIOUS HYPOTHESES 279 



nisms of the oxygen effect, however, the specificity of the poison would 

 appear to be exceedingly important; otherwise an unequivocal interpre- 

 tation of the results is not possible. Effects due to cyanide might result 

 from an inhibition of the cytochrome system, thus preventing the uti- 

 lization of oxygen by the respiratory enzyme systems of the cell. If 

 such a utilization of oxygen is necessary to bring about its effect on 

 radiosensitivity, then it might be expected that cyanide would decrease, 

 rather than increase, radiosensitivity. There is, in fact, one report, that 

 of Bacq et al. (3), that cyanide exerts a protective action against the 

 killing of mice by x-rays, but these results are not confirmed in similar 

 experiments by Dowdy et al. (9) with rats, in which a clear effect of 

 anoxic anoxia was demonstrated. With respect to the production of 

 chromosomal aberrations, it appears probable that the oxygen effect is 

 produced by oxygen dissolved in the aqueous medium of a cell, but ad- 

 ditional experimental evidence on this point is being sought. 



There is also the possibility that, even though H2O2 may be one of 

 the essentially primary radiation products associated with the oxygen 

 effect, it still may not be the actual mutagen directly responsible for 

 chromosome breakage. It may be only an intermediate in the formation 

 of other substances such as organic peroxides, some of which have been 

 shown by Dickey et al. (8) to have marked mutagenic effects in Neuro- 

 spora. It appears likely that the effect of organic peroxides may result 

 from free-radical formation; and, indeed, it is possible that most, if not 

 all, chemical mutagenic effects may be explicable on this basis and thus 

 turn out to be fundamentally related to radiation-induced mutations 

 [Auerbach (2), Dickey et al. (8), Jensen et al. (16)]. 



The Relation of the Oxygen Effect to Previous Views on 



THE Mechanism of Chromosome Breakage in Tradescantia 



BY Ionizing Radiations 



The preceding discussion has indicated the remarkable effect of oxy- 

 gen in increasing the radiosensitivity of Tradescantia chromosomes. 

 This effect can be most easily interpreted as resulting from an increased 

 production of chromosome breaks by x-radiation, the amount of increase 

 being positively correlated with the amount of oxygen present in cells. 

 On the basis of such results, it would appear that the previous hypothe- 

 sis utilized to explain the production of aberrations in Tradescantia must 

 be modified. On this hypothesis, as outlined earlier, chromosome break- 

 age has been considered to result from the direct action of the radiation 

 in ionizing the molecules actually composing a chromosome, as a con- 

 sequence of the passage through the chromosome of an ionizing particle 



