Discussion of Session II 



BERNARD S. STRAUSS 



The University of Chicago, Chicago, III. 



It is evident now that mutation is a complex process; the inter- 

 mediate steps depending on the particular mutagen used to 

 initiate the process. As Doctor Auerbach points out, we interpret 

 the action of mutagenic agents in terms of what we know about the 

 structure of the genetic material rather than learning about the 

 genetic material as a result of studies of mutagenic agents. This 

 process of interpretation may lead to useful results even though 

 its presuppositions are incorrect. 



In 1939, Thorn and Steinberg (16) 1 reported that nitrous acid 

 induced genetic changes in fungi. Their experiments were based on 

 the idea that the genetic material was protein in nature and their 

 interpretation was based on the well-known reaction of nitrous 

 acid with the amino groups — of the amino acids. Nitrous acid 

 is one of the more useful of the mutagens and its action is interpreted 

 today on the basis of its reaction with the amino groups — of the 

 purines and pyrimidines — since we now know, or feel sure that we 

 know, that genetic material is polynucleotide in nature. I think 

 Thorn and Steinberg should be given more credit for having first 

 studied the mutagenic action of nitrous acid and I think also that 

 it is instructive to note the pitfalls a favorite hypothesis can create. 



Nitrous acid-induced mutation represents a direct type of 

 mutagenesis. The agent produces a base analogue in situ which 

 can then yield a slight increase in the rare frequency of pairing 

 mistakes during DNA duplication. Ultraviolet light-induced muta- 

 tion as described by Doctor Haas probably represents the other 

 extreme, a complex of photochemical and biochemical events being 

 required for the complete mutation process. (In the discussion, 

 Doctor Atwood suggested that ultraviolet light-induced mutation 

 might be due to an in situ photocatalytic hydroxylation of a pyrimi- 

 dine DNA constituent. Since ultraviolet light does not cause muta- 



a See References, page 178. 



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