64 



J. A. V. Butler 



up in the fact that heat denaturation occurs more easily after 

 irradiation (Fig. 4). This effect occurs both in solution and 

 in the solid state, since it has also been found by Cox and 

 co-workers (1955), by titration curves, that X-ray treatment 



loo 



90 



80 



70 



c?60 



-I 

 I 50 



s^ 40 



©- 



30 

 20 

 lO 



^ 



20 30 40 50 60 70 

 TEMPERATURE 



80 90 IOO°C 



Fig. 4. Effect of irradiation on the temperature required to 

 denature DNA in 01 per cent aqueous solution. 



Unirradiated DNA. 



/\ Irradiated with 10^ r of 15 Mev electrons. 



......... Irradiated with 4 X 10« r of 15 Mev electrons. 



(Measurements by R. H. Pain on materials irradiated by Prof. 



Rotblat.) 



of solutions with doses of 8000 r causes a considerable break- 

 age of the hydrogen bonding. It is clear that this is a fairly 

 efficient process. 



When we ask what biological effects are produced by these 

 changes we are in a much more difficult position. If DNA is 



