230 



CELL HEREDITY 



20 25 30 



Temperature, °C 



330 335 340 



Reciprocal of Absolute 



Temperature (x 10"'') 



345 



FIGURE 8.18. The temperature relation of mutation from his to his^ in E. coli 

 (from Ryan and Kiritani, 1959, J. Gen. Microbiol., 20:644). 



the latter were so, the threshold of the temperature relation, instead 

 of being above zero, should be absolute zero ( — 273°C), at which tem- 

 perature molecular agitation is nil. The existence of a temperature rela- 

 tion does not entirely preclude the possibility that natural radiation is 

 the eventual cause of mutation because its effects are indirect, acting 

 through intermediate chemical reactions. Actually, this possibility is ex- 

 cluded for most mutations by quantitative comparisons of the frequency 

 of radiation-induced mutation with doses of radiation measured in the 

 environment. Less than 0.1 per cent of mutation in Drosophila can be 

 accounted for in this way, although in man, whose life span is long, a 

 fair proportion of all mutations may be induced by natural radiation. 

 We may conclude that some extragenic chemical processes limit the rate 

 at which spontaneous mutation occurs. The same can be said for in- 

 duced mutation. The chemical processes inferred may produce intra- 

 cellular mutagens or they may be only those involved in the synthesis 

 of the purine and pyrimidine bases, of the nucleosides and nucleotides, 

 and of the polynucleotides themselves. 



