MUTAGENESIS 87 



that way, at survival of the order of 10"^. In this case, it was most 

 probable that at that low survival rate, all the second strands and other 

 nuclei have to be destroyed. 



Auerbach: That is very drastic treatment. In Kaudewitz' experi- 

 ments, survival in some series was 30 per cent or higher. 



Zamenhof: I don't know about this case, but there are situations 

 where one can be sure that all that remains is just one strand of DNA, 

 because otherwise, one could not detect auxotrophs at all. 



Auerbach: If you work on the model that it acts on one strand 

 originally, but couldn't heat act on the whole double helix? 



Zamenhof: Yes, heat does, but it would not produce this particular 

 auxotroph in the second strand. What do you say, Josh? Wouldn't it 

 be that, if you detect an auxotroph without intermediate cultivation, 

 all the second strands of DNA and the other nuclei must have been 

 destroyed? 



Lederberg: Well, destroyed, or not giving a large clone. I agree with 

 the statement though. 



Auerbach: But couldn't there be treatments which affect both strands 

 at the same level, with heat? 



Zamenhof: That is unlikely to be a rule. It could be an exception, a 

 very rare exception. If you have 10 per cent auxotrophs it would be 

 very unlikely that you hit both spots on two strands in all 10 per cent. 



Neel: These are not single phage infections of bacterium; each bac- 

 terium is infected by multiple phages, is it not? 



Freese: No, by single phages. 



Lederberg: The bacterium is far too complex a genetic entity to do 

 experiments on mutagenesis with it. I don't think we have enough in- 

 formation on the genetic structure of the bacterium to go into the 

 chemistry of mutagens with it. Quite apart from the steps between 

 putting the chemical into the cell and the time it reaches the DNA, the 

 organization of DNA itself is certainly very complicated. You talk 

 about double-stranded DNA; it must be single-stranded at certain 

 stages. I have always been puzzled by the fact that with every dose, 

 with everything we have tried in inducing mutations, we always get 

 a mixture of mosaics and intact mutants. I think this has been every- 

 body's experience. You don't know that you don't have an equally 

 large number of mixed colonies, that you don't pick up as mutants in 

 your system, Stephen. 



Zamenhof: That is true. 



Neel: But if they were auxotrophs, they would be overgrown by — 



Lederberg: I agree, but, in addition to the ones that are intact, by 



