MUTATION AS A CHEMICAL PROCESS 



225 



tioned above could be in time only, not in space, so that the gene, upon 

 absorbing a quantum of ultraviolet light, might mutate after an interval; 

 whether it did or not might depend upon the condition of the cell both 

 before and after radiation. As a matter of fact, we must consider both 

 mechanisms possible. 



THE METABOLIC CONTROL OF MUTATION 



As an example of the indirect action of radiation, we have the observa- 

 tion that the effect of ultraviolet light can be reversed by visible light 

 iphotoreactivation). For every dose of ultraviolet light which is followed 

 by light reactivation and thus results in a certain number of survivors, a 

 lower ultraviolet dose can be found which, in the absence of light reac- 

 tivation, yields the same number of survivors. This is called the princi- 

 ple of dose reduction and is illustrated in Figure 8.15. The action 

 spectrum of photoreactivation has not led to a discovery of the cellular 

 component that absorbs the visible light, but it is known that the visible 

 light must be applied before DNA synthesis begins. This polynucleotide 

 requires for its synthesis other macromolecules such as protein. 



Some of the evidence for this conclusion comes from a study of bac- 

 teriophage. After bacteria are infected by phage there is, as Figure 8.16 



FIGURE 8.14. Number of 

 /no mutants induced in uni- 

 nucleate asexual spores of 

 /no Neurospora by increas- 

 ingly large doses of ultraviolet 

 light. The data were collected 

 for two different /no strains 

 (from Giles, 1951, C. S. H. 

 Symp. Quanf. Biol., 16:283). 



100 200 300 



UV Dose (seconds) 

 (2537 A) 



400 



