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Discussion 



Davis: I don't quite understand what you mean in saying that methylcytosine 

 has to replace thymine. Are there not solitary pyrimidine places where the methyl- 

 cytosine may have replaced cytosine ? 



Chargaff : I have to refer you to recent papers with Dr. Shapiro in which we 

 discussed at great length the problem of the solitary methylcytidylic units versus 

 thymidylic and cytidylic units. What I had tried to point out is that you can look at 

 the formation of such a chain — whether it is double or single is immaterial in this 

 case — as being a two-stage process. First, through a sieve as it were, an equal 

 number of 6-amino and 6-keto compounds are selected. These guarantee the pair- 

 ing principles which I have mentioned before, but what goes next to each other 

 seems to be controlled by what I have called, for want of a better term, an exclusion 

 principle. For instance, you can say that in the case of bromouridylic acid in E. coli 

 much more occurs as solitary units between two purines than would have been 

 predicted. You only have to compare the total molar sums of solitary thymidylic 

 acid and of solitary thymidylic plus bromouridylic acids. In other words, there is a 

 principle operating which says that bromouracil likes to be between two purines. 

 This intruder has been segregated in the chain entirely differently from the way in 



