131 



CARTER: Yes. 



ALLEN: Other people say that free phosphate is not increased by 

 radiation. 



CARTER: That is right. 



ALLEN: Then what is the contradiction? 



CARTER: I don't think there is a contradiction on that point because 

 Dr. Barron has not looked for acid labile phosphate but Weiss does say that he 

 gets inorganic phosphate as well during this operation. On that point there is 

 disagreement. 



COHN: Do you mind if I raise a point that I wanted to raise about 20 

 minutes ago? That is, a certain way of looking at phosphorus or ribose. 

 Phosphorus is a more important constituent of the macromolecule than purine 

 and pyrimidine because it is a double link in the chain. There has been a ten- 

 dency by some to look on these as less important than adenine or even of for- 

 mate experiments. This may not be justified in terms of the macromolecules. 



ALLEN: Could you go over again the question of the molecular weight 

 of this irradiated material "> The viscosity is greatly decreased in solution, I 

 understand, but you say that other criteria of the molecular weight indicate no 

 decrease. What are those other criteria'' 



CARTER: Sedimentation. 



MAZIA: Isn't it true that when you have molecules of such high 

 asymmetry, the sedimentation depends only on the width not on the length'' 



CARTER: What it depends on is orientation in the fields just like the 

 problems of anomalous viscosity. 



MAGEE: You are talking about sedimentation velocity'' 



CARTER: Yes. 



MAGEE: Don't you make sedimentation equilibrium measurements? 



CARTER: You can but I don't know that anybody has good data on 

 nucleic acid, and that would be the information that would give you the import- 

 ant data here. 



There are just one or two points that I think may be added here, and to 

 be fashionable we have to draw the Watson-Crick model. The Watson-Crick 

 model is based upon no new evidence. It encompasses a lot of analytical data, 

 and I think the work that Chargaff did in determining the composition of nucleic 

 acid is among the most important features of these data. 



The DNA molecule is looked upon, based upon the data of X-ray dif- 

 fraction crystallography, as composed of 2 intertwined helices developed about 

 the same axis, and I believe it draws out something like this: the length of turn 

 is about 34 a- The feature that is somewhat new compared to the Pauling 

 diagrams is that the phosphorus groups are on the outside. Incidentally, they 

 say that a structure like this cannot be described for ribose nucleic acid be- 



