MUTATION AS A CHEMICAL PROCESS 231 



DIRECTED MUTATION 



Students of mutation have long hoped to understand the process suf- 

 ficiently well to bring it under control. Although all known mutagens 

 are effective against many genes, it has been found that most mutants 

 respond to some mutagens and not to others; usually the differences are 

 only quantitative. Chemical mutagens appear to act with different 

 degrees of directness that have not yet been sufficiently analyzed; none- 

 theless, the best examples of specificity come from studies of chemically 

 induced back mutation in a double auxotroph of Netirospora. Here both 

 genes studied are in the same nucleus and, therefore, are under condi- 

 tions as similar as can be experimentally achieved. As is shown in Table 

 8.2, the inositoUess locus is most responsive to ethylmethane sulfonate 

 and least responsive to bromethyl methane sulfonate. In the case of the 

 adenineless locus, the situation is almost the reverse. Such specific re- 



TABLE 8.2 



The Relative Specificity of Six Mutagens on the 



Double Inositolless-Adenineless Mutant of Neurospora 



(From Westergaard, 1957, in Wolstenholme and O'Connor, Ciha Found. Symp. 

 on Drii^ Resistance in Microorganisms, Boston: Little, Brown cS: Co., p. 280; 

 and Koimark, 1956, C. R. Trav. Lab. Carlsberg, Ser. Physiol., 26:205; and 

 Westergaard, 1960, Chemische Mutagenese, Abh. deut. Akad. Wiss. Berlin, 

 Kl. Med., No. 1, 1960, p. 30) 



Mutagen Mutation per lO'' Asexual Spores Proportion 

 inn.s"^ ad~^ ad^ /inos^ 



Ethylmethane sulfonate 11.3 



