180 MUTATION AND PLANT BREEDING 



memory is correct. In addition, Kaudewitz has shown that nitrite-treated 

 bacteria produce nonsectored mutants. Mutants arising as a result of 

 P-32 decay tend to be sectored. If one assumes that the Watson-Crick 

 structure as confirmed by the experiment of Meselson and Stahl repre- 

 sents the genetic material at all times, and further assumes that a muta- 

 genic agent affects only one of the two DNA strands, then all mutants 

 should be sectored. With chemical mutagens such as alkylating agents, 

 the sectoring should be more complex for reasons indicated in my dis- 

 cussion. 



Now Witkin's experiment (for lactose nonfermenters) is quite clear 

 and, yet, Witkin herself at the Montreal Congress reported experiments 

 indicating that certain UV-induced mutations were "fixed" only at the 

 time of gene replication. There appears to be a paradox and essentially 

 the same point raised here by Doctor Auerbach was raised by Marshak 

 in the discussion of Witkin's paper on sectoring. If U V acted on a "chro- 

 mosomal" level and affected both strands, there would be no sectoring. 

 Perhaps the paradox is due to our assumption that the genetic material 

 is always double-stranded. 



Witkin irradiated her material to a survival of 10~ 3 . At this survival 

 one expects about seven lethal hits per organism. Since this Symposium, 

 it has been shown by Marmur and Grossman that one of the effects 

 of UV is to tie the two strands of DNA together by covalent bonds. 

 Nitrous acid treatment has a similar effect. DNA treated with either 

 of these two mutagens does not consist of two strands which can sep- 

 arate merely by breaking hydrogen bonds. I think these findings will 

 lead to an explanation for the lack of sectoring. 



