MAURICE ERRERA 



In the case of the haploid strain, the alteration of a single genetically 

 indispensable ' site ' is sufficient to obtain a killing effect ; two such ' sites ' 

 must be destroyed in the case of a diploid strain. The same explanation 

 might be given to the radio-resistance of the B/r strain of E. coli as compared 

 to the wild B type. 



CONCLUSION 



As a conclusion to this discussion, the only clearly apparent immediate 

 effect of ionizing radiation appears to be on DNA metabolism, perhaps 

 through some alterations of the structure of DXA or nucleoprotein itself. 



This does not necessarily mean that the DNA-containing cell constituents 

 are more sensitive to ionizing radiations than others, but effects on DXA 

 constituents may perhaps be more conspicuous because of their controlling 

 role on cellular processes and perhaps their greater specificity. 



Conditions on which other cell constituents can be made limiting are not 

 yet well understood. We mentioned the hypothesis of Spiegelman that 

 micro-organisms grown in the absence of adaptive substrate lose their 

 capacity of forming adaptive enzymes, perhaps through the loss of some 

 specialized cell particle. Similarly, Lwoff observed that rapidly dividing 

 Euglena finally lose their chloroplasts. It becomes conceivable that under 

 certain conditions (starvation, rapid division, etc.) limiting conditions might 

 be obtained and could help to explain some of the conflicting results discussed 

 in this paper f effects on protein synthesis or respiration). 



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