MANNER OF PRODUCTION OF MUTATIONS 601 



radicals produced by an equivalent dose of X rays. Thus, something 

 else appears to take the place of activated water in mustard mutagenesis 

 and to give results which are the same in some important respects but not 

 in all respects. In view of all this, it is evident that the study of muta- 

 genesis should be carried on with due regard to all its aspects, in order to 

 enable the best progress to be made, rather than by separating radiation 

 mutagenesis as a field in itself, apart from chemical mutagenesis. 



There seems little reason for doubting that, in the production of some 

 of the mutations, even when ionizing radiation is used, the primary change 

 takes place actually within the genetic material which thereby becomes 

 changed. The case cannot yet be proved categorically, but the studies 

 which indicate the mutagenic effect of transmutation of a phosphorus 

 atom proper to the chromosome afford a probable illustration of it, and 

 so do those showing the similarity of nucleic acid or protein absorption 

 curves for ultraviolet between 2500 and 3200 A with the mutagenic effec- 

 tiveness curves over this range. Possibly the mutagenic action of X rays 

 on dry anaerobic material (Hollaender) and on deeply frozen material 

 (Faberge) afford other illustrations of it. In any event, however, much 

 still remains to be done in the study of the more "direct" mutagenic 

 effects since even in this case (as the evidence from the reparative effects 

 on mutagenesis by ultraviolet indicates) a chain of processes is involved. 



The evidence of the noncompoundness of the gene or, in certain cases, 

 the chromosome (Muller, 1927, 1928a, b, c), cannot be regarded as com- 

 pletely convincing so long as the possibility remains that the mutagenic 

 effect, when produced on one element, may automatically spread in some 

 way to neighboring ones. Mazia and Blumenthal's (1950) results, show- 

 ing a surprisingly extensive spreading of the enzyme inactivation induced 

 by ionizing radiation when the molecules are arranged in a compact film, 

 bring attention to this problem. Perhaps similar studies with ultraviolet 

 would enable a decision to be made since, if such a spreading fails to 

 result from ultraviolet treatment and if the mutagenic effects are never- 

 theless eciuivalent in this regard, it might be concluded that there is no 

 spreading of this kind in mutagenesis by ultraviolet and that therefore 

 the gene and chromosome are essentially unitary. 



As this discussion indicates, a separation can no more be made (except 

 arbitrarily) between problems of the nature of the gene and chromosome 

 and those of mutagenesis than between problems of radiation and chemi- 

 cal mutagenesis. Thus radiobiology must be intimately concerned with 

 fundamental genetics, and the latter in turn should continue to profit, 

 as it has so largely already, by the radiobiological method of approach. 

 Moreover, since so many of the so-called "general biological" effects 

 of radiation are actually, in large measure, resultants of genetic effects in 

 the cells studied, the studies of "radiation damage" can also give impor- 

 tant suggestions in the investigation of mutagenic mechanisms and of the 



