(a) Cross-linking 



X 



or ,i 



irradiation 



DNA ; twin molecule 

 maintained by stereo - 

 specific hydrogen bonds. 



(a) 



"Active" end of break 

 capable of combining 

 with another active end. 



A proportion of the 

 ionizations that occur in 

 the DNA produce a 

 break in one of the 

 chains. 



.1--^ 



^^ 



^ 





\<> 



^ 



\^ 



Two molecules join if 

 the active ends can ineet. 

 In DNA fibres swollen 

 with water the cross- 

 linking efficiency is 

 higher because the inole- 

 cules are more mobile. 





\ 



*o, 



"Active" end becomes 

 peroxidized and is no 

 longer capable of form- 

 ing a cross-link. (Rate of 

 diffusion of oxygen into 

 dry DNA is slow and 

 "active" ends persist for 

 many days because lack 

 of movement prevents 

 cross-linking.) 



break in each of the ad- 

 nucleotide units ajjart. 



an 



(1)) Main-chain Scission 



This occurs when there is a 

 jacent chains less than about 5 



This is produced by radiation : 



1. Every time a DNA molecule is traversed by 

 a-particle* (600 eV/double break). 



2. When a cluster of ionizations (or other high en- 

 ergy event) is formed by sparsely ionizing radiations 

 (S.'ib eV; double break). " 



3. When by chance two isolated breaks come into 

 juxtaposition. Form statistical calculation one 

 "double break" will occur for every 70 random single 

 breaks. This mechanism is responsible for main-chain 

 scission by the indirect action of H and OH radicals 

 formed in the water. 



* Some cross-links are produced at the same time as main-chain scission by a-rays 

 due to the relatively sparsely ionizing 8-rays. 



Fig. 5. — Changes in the macromolecular properties of DNA brought about by ionizing 



radiations. 



