INDUCED MUTATIONS IN E. COLI 



269 



because of the direct contact of the chemical with 

 the sperm; high concentrations of chemicals can 

 be used without killing the organism, since a lo- 

 calized part of the body rather than the whole indi- 

 vidual is treated. These advantages point to the 

 possibility of a chemical analysis of the processes 

 leading to gene changes as well as the analysis of 

 the processes involved in direct changes in genie 

 nucleoprotein. With such objectives in mind, the 

 search for and study of the mutagenic activity for 

 many compounds would most efficiently be investi- 

 gated by using the bacterial or fungus method first, 

 then the use of the aerosol method, and finally, if 

 necessary, the vaginal douche technic. 



Zamenhof: I wonder if Dr. Witkin would be 

 willing to comment on qualitative differences be- 

 tween spontaneous mutations and mutations in- 

 duced by chemicals, in addition to the quantitative 

 ones. As Dr. Witkin has pointed out, the spon- 

 taneous mutations do not seem to occur in the 

 non-dividing cells (see also Zamenhof, Genet. Soc. 

 Rec. 13: 41-42, 1944). The mutation ratio in this 

 case can therefore be defined as the number of 

 mutations per cell division. On the other hand, the 

 mutations induced by chemicals seem to take place 

 even in the absence of cell divisions; in this case the 

 mutation ratio can be defined as the number of 



mutations per cell per unit of time. Thus the two 

 phenomena seem to be different. What is your 

 opinion on this subject? 



Witkin : The fact that mutations can be induced 

 in resting bacteria by radiations and chemicals, 

 whereas spontaneous mutations seem to occur only 

 in dividing cells, certainly suggests an important 

 difference between induced and spontaneous muta- 

 tions, although the end product of the two processes, 

 the mutated genes, need not necessarily differ in a 

 fundamental way. Chemical or mechanical errors of 

 duplication, or slips in some metabolic cycle inactive 

 in resting cells, may be the only natural circum- 

 stances capable of bringing about mutations. Thus, 

 the failure of spontaneous mutation to occur in rest- 

 ing cells may result from the relatively inert and 

 quiescent state of the genie material, and of the 

 metabolic activities associated with cell division, 

 rather than to a greater degree of inherent stability. 

 Zero point mutations may be due to the direct 

 action of chemicals or radiations on the gene, or 

 more indirectly, to the stimulation of cellular 

 activities normally involved in the production of 

 spontaneous mutations. A careful comparison of the 

 properties of mutants arising spontaneously and by 

 induction may throw some light on this important 

 question. 



