76 Discussion 



the molecular weight and determine the number of main chain bonds 

 broken, so that one can get quantitative values for the radiation change. 

 From a series of measurements we find that for every 10-15 electron 

 volts deposited in the nucleic acid (or a G value of more than 5) one 

 break occurs in the main chain if the DNA is irradiated in the solid 

 state. If the DNA is irradiated in solution the G value in the same 

 reaction is of the order of • 1. In other words, direct action is about 50 

 times more effective than indirect action for depolymerizing DNA. In 

 the cell where the DNA is present at relatively high local concentration, 

 direct action will be much more important than indirect action as far as 

 depolymerization is concerned. This is relevant to the point made by 

 Prof. Mitchell (see p. 71) with regard to differences in the radiosensi- 

 tivity of tumours. 



Latarjet: Experiments which were carried out on a transforming agent 

 of Pneumococcus by Dr. Ephrussi-Taylor and myself are not in agreement 

 with what Dr. Alexander has just said. The purified transforming agent, 

 which is DNA, is tested not by physicochemical properties but by its 

 biological specific activity, and its sensitivity to radiation remains the 

 same whether it has been treated with 5 m urea or not. 



Haddow: I don't think that is necessarily an inconsistency. 



Alexander: The answer may be that DNA's from different sources 

 don't all behave in the same way towards urea. Herring sperm DNA is 

 dissociated, DNA from thymus is not dissociated until it has received a 

 further treatment (Alexander, P., and Stacey, K. A. (1955), Nature^ 

 Lond., 176, 162). 



