133 



important in the formulation of nucleic acid structure and action, and the data 

 we have are not very good on the number of end groups in one of these chams. 



TOBIAS: According to Stent and Schachman's idea, only one arm of 

 the chain is broken in irregular places and not as far as 2100 units but maybe 

 Z5 units. 



Stent thinks that half of the nucleic acid chainmight be broken in various 

 places along the nucleic acid molecule. When the molecule is irradiated, ion 

 pairs can cause other breaks along the chain, usually in a single arm only. If 

 a radiation-induced break occurs in a place where there is only a single chain 

 intact, the molecule is completely broken. 



When phage are irradiated at various temperatures, it turns out that 

 the radiosensitivity of the phage (and I think Dr. Pollard has done similar ex- 

 periments and similar experiments were done on cells by Wood as well) shows 

 a very striking increase with temperature. Sensitiyity may go up as much as 

 5 times within a few degrees centigrade. 



The interpretation there is that some of these hydrogen bonds, statisti- 

 cally speaking, break with increase in temperature, and Stent thinks that if 

 maybe 20 of them are broken simultaneously, then the defect is as effective as 

 a defect caused exactly opposite the already open places. This is really quite 

 an intriguing idea to me at least, though I must admit that the explanation of 

 thermal increase of radiosensitivity does not require the Watson-Crick model. 



CARTER: Actually I think that you have to implicate some kind of a 

 staggered structure to get these long chains. Do you feel that way? 



CHARGAFF: Yes, unfortunately there are no calculations available. 

 I think that you want to calculate how long a chain can exist without snapping. 

 There are limits to a continuous polymer chain of this type, but I can think of 

 no criteria. 



PLATZMAN: If you envisage that, the position has already changed 

 with time. It is a dynamic, changing thing. 



CHARGAFF: The desoxynucleic acid is really supposed to stay where 

 it is, in the resting cell. It does not turn over at all. 



PLATZMAN: Certainly these fluctuations in hydrogen bonds are 

 dynamic. 



CHARGAFF: I think a bridge builder would understand more about 

 the effect of these hydrogen bonds on 2 parallel chains than I do. But I have a 

 feeling that you have a mutual strengthening of 2 different types; (1) a covalent 

 type of linkage and (2) a secondary valence one. It is possibly true that if one 

 of the types breaks in several places, you get automatic snapping of the other 

 type. I think if you break a few covalent bonds, many of the hydrogen bonds 

 will be disrupted. 



CARTER: Incidentally, there is some supporting data from infrared 

 spectroscopy. Frick found in the 3. 1 - 3.2 micron region a shoulder which 

 corresponds with the nitrogen-hydrogen bonds. Upon titrating his nucleic acid 

 he found that when viscosity fell, the shoulder disappeared. 



CHARGAFF: I think the Watson-Crick structure describes probably 



