60 MUTATIONS 



Novick: In the work you refer to, we studied only caffeine-induced 

 mutations, not spontaneous (13). You will recall that in the presence 

 of chloramphenicol the synthesis of protein ceases and that of nucleic 

 acid continues. We showed that if caffeine is present with the 

 chloramphenicol, mutations are induced, these being detected by re- 

 moving the bacteria from chloramphenicol and plating them on ap- 

 propriate selective agar. If the rate of mutation is expressed per unit 

 of DNA formed, then the caffeine-induced rate is the same as in the 

 absence of chloramphenicol. These experiments show that the synthesis 

 of DNA is sufficient for mutations to occur and that the DNA formed 

 in the presence of chloramphenicol is good DNA. 



Auerbach: I would like to mention a very curious result that I 

 got, using Kim Atwood's technique for scoring lethal mutations in 

 Neurospora (2). I scored the spontaneous occurrence in desiccated 

 spores. The mutation frequency increased absolutely linearly for seven 

 months in these dried spores. It increased something like 1.3 per cent 

 per week. Atwood's system is very ingenious. It is rather like the CIB 

 method in Drosophila. You carry recessive lethals in a heterokaryotic 

 condition and force them into the open by giving the right supplement 

 for homokaryosis. 



At the same time, I ran a series at 4° which was not very good, 

 because here humidity was very poorly controlled. They did get 

 humid, and the line was not so straight. It wobbled a bit. The aston- 

 ishing thing was — but, unfortunately, I did it only once — at one point, 

 say after four months or so, I took a sample of spores which had been 

 sitting at 4°, put them at 30°, and left them there for, I think, a fort- 

 night. Then I tested for mutation, and the frequency had gone right 

 up to the level of the spores kept at high temperature. I want to repeat 

 this experiment because it raises the possibility that in the cold some 

 chemical mutagen is formed or some condition predisposing to muta- 

 tion takes place, and that only the last step in mutagenesis has the 

 observed high temperature coefficient. 



Atwood: At the 4° temperature, something happens that then softens 

 up whatever it is for mutation to occur later at 30°? 



Auerbach: No, there are two points here. This is the second, and 

 I'm not so sure about it because I did it only once, but it was very 

 striking. This curve shows that mutation occurs not only without cell 

 division but also without chromosome replication, because Atwood's 

 system is such that if, say, this is the double helix, and suppose the 

 mutation occurred here, it would give rise to a colony which is half- 

 mutated. Now, this is the only good system that I know of in which 



