120 MUTATIONS 



Goodgal: But do you mean to say that they are all independent? 



Demerec: Yes, they are independent. We took the necessary pre- 

 cautions to insure that all the mutants mentioned in Table 2 originated 

 independently of one another. 



Auerbach: The duplication creates a special situation. It is a means 

 for producing deletions. 



Demerec: Yes. I was just talking about the mechanism for induction 

 of mutants. I think that it is a loop formation, even in short deletions. 



Lederberg: But if you have a duplication that is functional, there 

 is a bias in favor of deletions, because the point inactivation would 

 have no phenotypic effect. 



Auerbach: Except the ones in the region between. 



Demerec: If the presumed duplication consisted of genetically func- 

 tional segments, a single-site mutation occurring within a duplicated 

 region could not be detected, and the observed number of single-site 

 mutants would be smaller than the actual number of mutations with 

 the result that there would be a bias in favor of deletions. If the dupli- 

 cated segment is genetically nonfunctional, however (a "nonsense" 

 region), and I am inclined to believe that it is, no such bias would 

 exist. In any case, even the existence of a bias would not affect the 

 conclusion that can be reached from the data. 



Freese: Has anybody looked at the induction of mutations after P^^ 

 incorporation? 



Demerec: No. It is a quite laborious process to get those. 



Freese: There might be a difference between phages and bacteria; all 

 those phages in which the chromosomes are broken are probably lost, 

 while in bacteria such broken chromosomes may possibly be saved by 

 some healing process. 



Zamenhof: One of the chemical mechanisms where short "deletion" 

 could occur is a reaction in which the sugar is destroyed. The agents 

 which will destroy the sugar should produce short "deletions." We think 

 that this may happen when you get "deletions" by heating in the dry 

 state. The reaction which occurs there is not always hydrolysis, where 

 you only split off the base, but it may be pyrolysis (84) ; there may be 

 a dehydration and the sugar may be lost. 



Demerec: This means that the sugar is lost and all the bases which 

 are attached to that part are lost. 



Zamenhof: If the sugar is lost, the nucleotide will never reproduce; 

 while in certain cases, heat could produce just the loss of purine, in 

 other cases, even in the case of pyrimidine nucleotide, if the sugar is 

 destroyed, the corresponding nucleotide would be completely lost. 



