316 GENE MUTATIONS CAUSED BY RADIATION 



looked for, and that there is no present way of relating this calculated 

 volume to the actual size of the gene. At the same time, however, they 

 did not yet concede the possibility, proposed by the present writer, that 

 a single "hit" might, like a bomb or shell, sometimes give rise to a 

 branched chain of reactions, so as to cause two spatially distinct yet 

 near-by genetic changes. 



Despite all the above discussions and publications, a number of other 

 investigators have since 1935 brought up the method again for the 

 finding of the size of the gene or chromonema, or of all the genetic ma- 

 terial in a chromosome. The attempts along these lines which have at- 

 tained most prominence in recent years have been those of Lea and 

 Catcheside (35, 3^). 



In view of the revival of interest which the last-named workers have 

 aroused in this matter it may be permissible to risk redundancy by 

 restating here that the target hypothesis, as applied to calculations of 

 the size of the gene or even of some supposititious "sensitive" portion 

 of it, must make the following assumptions, all of which have to be true 

 at once if the method is to work, but no one of which has yet had criti- 

 cal evidence adduced in its favor, (a) Virtually all the radiation- 

 induced mutations result from ionizations rather than mere activations. 

 (6) Virtually every ionization within a gene, or within its "sensitive" 

 portion, results in a mutation, no matter what the circumstances. 



(c) These mutations are (at least for the genes studied) all detectible. 



(d) No ionization or activation occurring outside the gene (or its "sen- 

 sitive" portion) can result in its mutation. That not all these postulates 

 are true at once is directly shown, as stated by the author (44) in 1937, 

 by the variation in gene-mutation rate under different conditions. The 

 more realistic task thus becomes that of throwing "light upon what 

 atoms and atom configurations are the more important ones in the in- 

 itiation of the mutation and breakage processes, and upon what kind of 

 steps may be involved in the secondary reactions occurring between the 

 ionization and the genetic change itself" [Muller (45), p. 46]. 



Lines of Attack on the Intermediate Steps of 

 Mutagenesis by Radiation 



The discussion in the preceding section has been in a sense negative 

 in its results, in showing the artificiality of postulates which have 

 hitherto been widely adopted, but this may be helpful in clearing the 

 track of investigation for studies of the actual physicochemical mecha- 

 nisms at work. 



The use of the chemical attack for an analysis of the steps involved 



