Discussion 



75 



If the measurements are made in dilute salt solution, quite high doses 

 are required to decrease the apparent molecular weight. When un- 

 irradiated herring sperm DNA is dissolved in concentrated urea solution, 

 its molecular weight is halved, and is now only 3 million instead of 6 million 

 (Alexander and Stacey, (1955), Biochem. J., 60, 194). If the irradiated 

 DNA is measured in solution containing urea then relatively small doses 

 produce a decrease in molecular weight. A typical experiment is as 

 follows : a sample of DNA containing about 5 per cent of moisture is 

 irradiated with 2x10^ rad and its molecular weight measured dissolved 



IRRADIATION 



a 



Fig. 1. (Alexander). A diagrammatic representation of 

 the effect of urea on irradiated DNA which shows how 

 breaks hidden in the dimeric structure (b) become ap- 

 parent in urea solution when all the hydrogen bonds 

 between the chains have been broken. 



either in salt or in urea solution. In salt solution the average molecular 

 weight of the sample is hardly changed (6-5 x lOHo 6-0 x 10^), while 

 in urea solution the average molecular weight is more than halved. 

 We have interpreted this as follows : in urea solution the herring sperm 

 DNA molecule is dissociated and the two parts move freely; hence any 

 breaks introduced in any one chain will be revealed by molecular 

 weight measurements. When the material is measured in salt, the two 

 parts do not exist independently and two breaks have to occur in fairly 

 close juxtaposition in the two chains before a break can be detected by a 

 physicochemical measurement. Fig. 1 shows diagrammatically what we 

 believe to happen. Because of the dimer structure of DNA some of 

 these breaks are hidden and are only revealed when the structure is 

 opened out. With light scattering one can makf direct measurements of 



