88 MUTATIONS 



my own experience and every other experience that I know of, at any 

 rate, there are many mottled or mixed colonies as well. 



Zamenhof : Oh, yes, among the remaining colonies which could not 

 be picked as auxotrophs. 



Lederberg: Well, I think, too, there is not that much consistency in 

 Witkin's data in this respect (80). I agree with you that the matter is 

 extremely puzzling. I think that it reflects considerable ignorance as to 

 what the details of genetic structure of bacterium are. You have a 

 bacterium in different states of division at the time of the treatment, 

 and I think we can assume that some DNA will be well compounded, 

 that it will be in the double strand. We just do not know how homo- 

 geneous the different specimens are in terms of the state of the DNA 

 at the time of treatment. Spores are, perhaps, a more favorable mate- 

 rial than E. coli because it is at least likely that they are mainly 

 uninucleate, whereas, in the case of an E. coli suspension, you have 

 a very different situation. One cell is very different from the next 

 one. 



Auerbach: I had the same problem as Dr. Zamenhof when I scored 

 spontaneous mutations in Neurospora, as I described yesterday. I felt 

 the same way; we just know too little about the nature of the genetic 

 material in Neurospora. These were dried spores and spontaneous 

 mutations, so there was no drastic treatment. 



Magni: I want to go back to Dr. Auerbach's last point on the two-hit 

 inactivation curves with nitrous acid in bacteria. We also found two- 

 hit inactivation curves with the same mutagen in resting haploid yeast 

 cells which are certainly not binucleate. 



Auerbach: How much does this depend on conditions? I am think- 

 ing of the work they do in the Hammersmith Hospital in London, 

 particularly Alper (2), who finds in bacteria that the target number 

 depends not only on conditions of treatment of bacteria, but also, for 

 instance, on the plating medium; so that it is difficult to know what 

 the target number is. 



Magni: This is true, according to my experience for bacteria, but not 

 true for yeast. We never found a multihit curve on haploid yeast with 

 X-rays, changing the plating conditions. 



Auerbach: After nitrous acid, I want to go on to experiments with 

 purine and pyrimidine analogues. Here I have a very specific question 

 to ask, but I should like to preface it by a few points that puzzle me. 

 There is a presumption, a very plausible one, that broraouracil acts by 

 being incorporated into DNA instead of thymine. There are a number 

 of points which make me doubtful of this. 



