60 J. A. V. Butler 



very stable one. It is disrupted by heat in water or salt solu- 

 tions, and is also sensitive to the action of dilute acids or 

 alkalies. Heating (e.g. at 100° for 15 minutes in water) causes 

 a great decrease in viscosity of solutions of DNA, with no 

 very marked change of sedimentation behaviour (Zamenhof, 

 Alexander and Leidy, 1954; Doty and Rice, 1955). There 

 have been conflicting interpretations of this, due mainly to 

 differences of conditions. However, there is no doubt that a 

 considerable amount of disruption of the hydrogen-bonded 

 structure occurs on heating, with a decrease in molecular 

 weight which depends partly on the specimen used and partly 

 on the concentration. On the basis of their results Dekker and 

 Schachman (1954) have suggested that the nucleotide thread 

 is interrupted at various points and that the DNA particle is 

 held together by hydrogen bonds between the overlapping 

 segments. It is, of course, difficult to establish whether such 

 interruptions are (1) originally present in the DNA; (2) pro- 

 duced during the preparation by the DNAse present in the 

 cells; (3) not originally present, but only caused by heat. The 

 drop in molecular weight (1/4 — 1/6) on heating was found by 

 Shooter, Pain and Butler (1956) to be much less in good speci- 

 mens than that found by Dekker and Schachman. It has also 

 been shown by Shooter and Butler (1956) that degradation 

 occurs at quite a rapid rate in the cell homogenates and even 

 in isolated (aqueous) nucleoprotein. The third possibility 

 would imply "weak points" in the nucleotide at which dis- 

 sociation by heat occurs, which might be the case if a few of the 

 PO4 bonds are triply esterified and thus easily hydrolysed. 

 No independent evidence of this has been obtained. However, 

 comparable heating in the solid state produces no degradation, 

 so that the eff'ect of heat may involve hydrolytic changes. 



The effect of ionizing radiations on DNA has been studied 

 under a variety of circumstances. In aqueous solution (0-1 

 per cent) the characteristic high viscosity of DNA is greatly 

 reduced by comparatively small doses of radiation (see Fig.l), 

 about 8000 r being required to reduce the intrinsic viscosity 

 by one half (at 0-1 per cent). This is due mainly to a 



