84 MUTATIONS 



When we extrapolate this straight line to the origin we do not get 

 the value 1, as we would expect if all the mutants induced are mosaics, 

 but we get about .7 to .8. This probably means that even without 

 mutagenic treatment a certain percentage of one of the two DNA 

 strands are thrown away in the bacterium. That is at least one ex- 

 planation of this observation. 



Lederherg: Couldn't you already have 30 per cent of spontaneous 

 lesions in one strand? In other words, you are extrapolating back to 

 zero external dose or intrinsic dose of similar lesions already present. 

 This is not different from your statement, but it has the somewhat 

 different connotation that this per cent of losses could be an accident 

 and not a determinant. 



Freese: That's right, it could be. 



Goodgal: You would conclude from this that both strands of the 

 DNA are active? 



Freese: Well, I think you have to be very careful with the word 

 "active." The observation of mottled plaques indicates that both 

 strands of the DNA give rise to progeny phages. It does not follow 

 that both strands are functionally active themselves. 



Goodgal: No, but genetically. 



Freese: Yes, genetically. They can give rise to mottled plaques. 



Auerbach: Can one see that they are half and half? 



Freese: You can isolate the phages from the mottled plaques and 

 replate them ; then you get varying mixtures of r and wild-type phages. 

 It is not half and half. The ratio varies. 



Auerbach: That may be just genetic drift or selection. 



Freese: Well, there are at least two factors. You have selection in a 

 plaque and selection in a bacterium. Some of the phage genomes may 

 duplicate faster or more often than others. Some may mature earlier 

 than others. So you have some variance there. 



Novick: There was some confusion in the discussion of this figure, 

 or it confused me. The origin is infinite dose, isn't that correct? — that 

 is, dose decreasing as it goes on. 



Freese: No, the origin is zero dose. The abscissa is per cent of 

 mutants induced. 



Novick: I see. You start out, then — 



Freese: I start out with no mutants induced. 



Novick: Oh, you treat r and get r + , r+ to r? 



Freese: I go from r-\- to r. 



Auerbach: Don't you get any spontaneous mutants? 



Freese: The spontaneous mutants are negligible as soon as I plot the 

 first point on this curve. 



