MANNER OF PRODUCTION OF MUTATIONS 587 



selective absorption. This conclusion is concurred in by Powers (unpub- 

 lished, cited by Powers and Shefner, 1951). The inference has therefore 

 been drawn by Rubin and by Powers that the excess frequency of muta- 

 tions, above that explicable by ionizing radiation, was caused by the 

 changes in atomic nuclei. 



Tritium (H^), in contrast, gave no evidence of mutagenesis in Para- 

 mecium except via ionizing radiation (Powers and Shefner, 1948). This 

 corresponds with the considerations that only a small part of the hydro- 

 gen in the cell is in its chromatin and that even the hydrogen which is in 

 the chromatin is, for the most part, much more readily replaceable, if lost 

 or transmuted, than is the phosphorus in it. 



In work on mutations to streptomycin resistance in E. coli, caused by 

 P^-, the frequency of mutations expected from the ionizing radiation was 

 taken into account by Rubin (1949; see also Rubin and Steinglass, 1949, 

 for an earlier, less definitive treatment of the matter). Here it could be 

 shown that, on account of the very small size of the bacterial cells and the 

 comparatively long range of the given ^ rays, the genetic material must 

 have been exposed to practically the same amount of ionizing radiation 

 regardless of the distribution of the P^^ between the cells and the medium. 

 This being the case, the fact that significantly more mutations were pro- 

 duced when there was a smaller amount of the stable P^^ accompanying 

 the P^^ as carrier and when more P^^ was therefore present within the cell 

 and its chromosomes was taken as showing that a significant proportion 

 of the mutations was in that case produced by the transmutations in the 

 nuclei of the intracellular atoms. In line with these conclusions are those 

 of Hershey et al. (1951), who have shown that in phage the killing effect of 

 P^^ is far greater than can be explained by the ionizing radiation produced 

 by it. They calculate that approximately 1 in every 12 transmutations 

 of phosphorus located within the phage gives rise to its death as a result 

 of the atomic nuclear change or recoil itself. 



It should be noted, however, that neither in this case nor in the case of 

 mutagenic changes is there yet a secure basis for inferring that the effec- 

 tive transformations were always in atomic nuclei that had formed a part 

 of the actual genes. For chemical changes in other substances, e.g., those 

 which later took part in the formation of daughter genes, might also be 

 conceived as having a mutagenic effect. For this reason it would not 

 be safe to conclude, even if the evidence for mutagenesis both by trans- 

 mutation of phosphorus and of sulfur were admitted to be convincing, 

 that both the nucleic acid and the protein components of the chromosome 

 are to be regarded as participating in the structure of the actual gene. 



It is to be expected that, if transmutations occurring in administered 

 radioisotopes which have become incorporated in the cell can cause muta- 

 tions, then transmutations caused by the absorption of slow (thermal) 

 neutrons would be able to do so also, provided that the atoms involved 



