314 P. Alexander, S. F. Cousens and K. A. Stagey 



separated and this can be done with DNA from herring sperm 

 (Alexander and Stacey, 1955a and h; 1957a), by exposing a 

 dilute solution to 4m urea when the molecular weight is 

 halved, from 6 X 10^ to 3 X 10^ without affecting the length of 

 the molecule. After irradiation with X-rays the molecular 

 weight of the DNA when measured in salt falls much less 

 rapidly than the molecular weight determined after dissocia- 

 tion by urea, because eight out of ten of the breaks occur in 



IRRADIATION 



UREA 





N 



I 1 



a D c 



Fig. 10. A diagrammatic representation of the effect of 



urea on irradiated DNA which shows how breaks hidden 



in the dimeric structure {h) become apparent in urea 



solution when all the hydrogen bonds are broken. 



one of the chains only and remain hidden until the molecule 

 is split* (Alexander and Stacey, 1956). When a-rays are used 

 no hidden breaks are produced because the destruction along 

 the track of the particle is so intense that both chains are 

 invariably broken when crossed by an a-particle (Stacey and 

 Alexander, 1957). 



Breaks in the chain are also produced by the alkylatincr 



* The double breaks are not due to a chance event whereby two ionizations 

 are produced close together in each of the two chains. They are probably 

 produced by the densely ionizing tail portion of each electron track. 



