INITIAL RADIATION DAMAGE AT SUB-CELLULAR LEVEL 13 



standing and takes up a more coiled configuration. This change is 

 accompanied by a fall in viscosity. This type of denaturation 

 occurs more readily if there are "hidden breaks". Hence if DNA is 

 dissolved at ionic strengths of less than 10-2 M the molecule coils 

 up, denatures slowly and this coiling leads to a reduction in vis- 

 cosity after irradiation. This type of after-effect is completely pre- 

 vented if DNA is irradiated at ionic strength approaching physio- 

 logical. 

 2. DNA is very susceptible to oxidation by dissolved chlorine which 

 produces breaks in the main-chain in the same way as OH 

 radicals do (Moroson and Alexander, 1960). If DNA is irradiated 

 in solutions of sodium chloride, some of the OH radicals react to 

 give dissolved chlorine. This will then react slowly with DNA to 

 produce breaks (Alexander, 1959). The addition of sodium thio- 

 sulphate immediately after reaction prevents this type of post- 

 effect by removing the dissolved chlorine. If steps are taken to pre- 

 vent these two reactions the fall in viscosity following irradiation 

 is small and represents less than 20 per cent of the total 

 change. 



Direct action by sjxirsely ionizing radiations. The changes produced 

 wdien DNA is irradiated in the dry state or as a concentrated gel are 

 extremely complex and the observed effects depend critically on the 

 experimental conditions (Alexander etal, 1960; Lett et al, 1961a, b). 



DNA containing 20 per cent of moisture or less is almost unaffected 

 by irradiation and the presence of oxygen has relatively little effect. 

 DNA fibres containing an equal weight of water are readily degraded if 

 irradiated in the presence of oxygen but become cross-linked when 

 irradiated in its absence. Cross-linking is recognized by an increase in 

 molecular weight and at higher doses the formation of an insoluble gel. 

 We have explained these findings by postulating that the result of an 

 ionization is to produce a break of one of the chains in such a way that 

 one of the ends is active (e.g. a free radical) so that it can combine 

 with another end to produce a cross-link. The active ends combine 

 readily with oxygen after which they are no longer capable of cross- 

 linking. In the dry DNA, diffusion of oxygen is slow but molecular 

 movement is also limited and few of the active ends have an oppor- 

 tunity to form cross-links and the majority are wasted. When the DNA 

 is swollen with water the opportunity for interaction betw^een active 

 ends is greater, but there is now a competing process due to the rapid 

 penetration of oxygen. This complex set of reactions is summarized 

 iji Fig. 5, 



