MUTAGENESIS 107 



75 per cent of the mutants are due to transitions. That means they are 

 due to base pair changes from AT to GC or vice versa, and, in fact, 

 from GC to AT. 



Benzer: What you have is a gap. 



Freese: However, we would expect to get some changes in which a 

 purine is replaced by a pyrimidine and vice versa. 



Benzer: Shouldn't those be equally probable? You have a not-base, 

 and it doesn't pair with anything. 



Freese: I reason from the experimental facts which we have observed 

 with the low pH treatment — 



Benzer: But you are assuming that the other one works by the same 

 mechanism. You should make a prediction on this thing by itself. 



Freese: I can give you an explanation, which would be that there 

 are more pyrimidine precursors than purine precursors, so that across 

 a gap, pyrimidine gets easily incorporated into the DNA, and the 

 purine does not. But this is only an explanation of the observations. 



Lederberg: Does the ethylethane sulfonate also reverse the mutants? 



Freese: No, this we haven't done so far. The reference here was to 

 ethylmethane sulfonate for which Green has found that he could not 

 induce the reversions of these mutants. 



Novick: You find that EMS-induced mutations are not reversible? 



Freese: By EMS. 



Novick: What about BU? 



Freese: We haven't done that so far. 



Zamenhof: Also, heat-induced mutations are not reversed by heat. 

 They may reverse spontaneously but not by heating. 



Benzer: Is the prediction sufficiently flexible that it will fit any 

 experimental result? 



Freese: With respect to transitions and transversions, yes. I cannot 

 predict whether you get more transitions or transversions, but when 

 you look at the reverse mutations, you predict that since we attack 

 the guanine, we get preferentially changes from GC to AT. As long 

 as you stay in the framework of transitions, I make a prediction. But 

 how many transitions and transversions, I wouldn't dare predict. 



Goldstein: Why couldn't you predict that the ethylethane sulfonate 

 mutations would only be reversed by aminopurine? If the result of the 

 gap is to pair C, you don't have a mutation, because you have the 

 same GC pair that you started with. If the effect of the gap is to pair 

 G, you have CG in place of a GC, and this still should not be affected 

 by bromouracil, according to your theory. 



Lederberg: No, that is a mistake in the premise. To be consistent 



