72 Discussion 



Lajtha: We get about 12| million i*C atoms incorporated into DNA! 



Butler: You would require a fairly heavy dose in order to produce 

 detectable liberation of adenine. I think even in solution you require 

 fairly heavy doses, about 100,000 r ; 8,000 r will produce perhaps one 

 break in nucleotide chains, a very small fraction of the whole. With 

 regard to the other questions, these involve synthetic reactions and I 

 don't know what the synthetic reactions are in the mechanism of 

 synthesis of DNA. It is true that radiation has a marked effect on 

 spleen and thymus, and it has been reported that the DNA obtained 

 from the radiated spleen and thymus is relatively broken down. We 

 tried to repeat that, but we were not able to detect any difference in the 

 isolated DNA. What happens, of course, is that the radiation kills the 

 cells and you may get DNA which has been metabolically damaged by 

 enzymes from dying cells. 



Forssberg: In vivo irradiation sometimes causes a marked degradation. 

 Bachmann and Harbers irradiated Walker carcinoma with 5,000 r and 

 isolated two DNA fractions according to Bendich, DNAi having a high 

 and DNAii a lower molecular weight. In the non-irradiated material 

 there was about three times as much high molecular DNA as low 

 molecular; but after irradiation with 5,000 r, within two hours the ratio 

 was about • 2-0 • 3 ; so there is a very high grade depolymerization in this 

 carcinoma. Also the incorporation rates of isotopes are different in 

 these two fractions. 



Gray: I would like to ask Prof. Butler for his observations on the 

 paper by Dr. Kaufmann and his group which seemed to me very inter- 

 esting as stressing the great sensitivity of the nucleoprotein as distinct 

 from DNA. They performed several experiments. In the first series of 

 experiments they formed a gel from calf thymus, irradiated the gel 

 with 1,000 r and then studied the fall in viscosity. This was much more 

 rapid than when they had high salt concentration present which dis- 

 sociated the nucleoprotein. In another experiment the effect of radiation 

 on a dilute solution of an artificial nucleoprotein obtained by mixing 

 calf thymus DNA with bovine albumin was compared with the effects of 

 radiation on dilute solutions of the DNA and the protein separately. 

 After exposure to 1,000 r the fall in viscosity was very much greater in 

 the former case, that is, when the nucleoprotein was irradiated. It is of 

 interest that the DNA used in these experiments was derived from calf 

 thymocytes and I wonder whether these experiments provide a clue to 

 the rather remarkable radiosensitivity of these cells. 



Butler: This is rather reminiscent of Anderson's experiment at Oak 

 Ridge. The only thing that one can say about it is that you have there 

 a complex, a gel-like system of filaments which are bound together in 

 some way, and it certainly is true that very small doses of radiation 

 break down this structure. However, it is a very labile binding, because 

 it is affected even by shearing forces. What the mechanism of it is I don't 

 know; Dr. Hollaender may. 



Hollaender: This nucleoprotein, which Anderson has prepared from 

 thymus, spleen, and other organs, responds readily to as little as 25 

 roentgens and is almost like water (Fig. 1). It behaves in many ways 



