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KAMEN: The ratio of purine to pyrimidine has never been 1, exactly. 



MAZIA: Pauling's model deals with DNA. It is a fact that the ratio of total 

 purines to total pyrimidines in DNA is unity. This has been shown for many 

 kinds of DNA. What varies is the ratio of one purine to the other, with a cor- 

 responding variation of the ratios of the pyrimidines to each other. 



ONSAGER: There also would be the question of whether there would be as 

 much of one purine as there would be of its matching pyrimidine. They can fit 

 themselves pretty well by sets of two hydrogen bonds. 



FANO: Could you explain to which one is the purine and which one is the 

 pyrimidine and which is the matching? 



ONSAGER: I don't know exactly how they go. They are just drawn like this 

 and there will be two places for the hydrogen bonds. 



MAZIA: James and I have measured the thickness of monolayers of DNA. 

 We consistently come out with a value of about 22 A, which is just about 1. 5 

 times the long dimension of a nucleotide. We could not explain our results in 

 terms of a single nucleotide chain, but the newer concepts of helices composed 

 of more than one chain fit the measurements very nicely. I have consulted with 

 Pauling about these film measurements, and he feels that they are consistent 

 with the idea, which Dr. Onsager has just described, that when we speak of a 

 DNA molecule we refer to a unit consisting of more than one chain. 



FANO: Can you explain which dimension you are talking about? 



MAZIA: The "long dimension" I referred to is the distance from the "edge" 

 of the base, through the sugar, to the "edge" of the phosphate. The space oc- 

 cupied by a nucleotide is often pictured as an elliptical disc. The long axis 

 of the ellipse is about 13.5 A. We were not able to determine, in our mono- 

 layer studies, whether there was any preferential orientation of the base end or 

 the phosphate end toward the water. 



The measured thickness of the DNA monolayer is one and one half times 

 the maximum thickness predicted for a single nucleotide chain. Pauling cal- 

 culates the cross section of his helix and it comes out to the same value as the 

 thickness of the monolayer -- 22 A. 



KAMEN: What about the protein? Where does it go? 



MAZIA: I don't see how Pauling's molecule can react with protein or any- 

 thing else that combines with DNA through the phosphate residues. He places 

 the phosphates on the outside of a tightly packed structure. 



POLLARD: He thinks it can be opened enough to include the protein. He 

 does not think with straight nucleic acid he can do it but he thinks with protein 

 he might. At least he did two weeks ago. 



ONSAGER: The model would seem to suggest that the blue print is just 

 taken out of the safe when it is needed. 



MAZIA: That can hardly be so. We know that the blueprint is in use all of 

 the time, although at a slow pace. We know this from the results of simple ex- 

 periments where we remove the nucleus from a mature, fully grown cell. This 

 cell cannot live for very long without its nucleus, and, so far as we know, the 



