W. D. JACKSON 



The last decade has seen the replacement of this concept by the view that 

 chemical and physiological principles are deeply involved with the physical 

 phenomena. In particular it was shown that radiation sensitivity varied 

 widely both in organisms and in individual cells, depending on development 

 and on physiological conditions'^. The discovery of the modifying effect of 

 oxygen was an important step. Following Thoday and Read's finding'^ that 

 oxygen increased radiation damage in bean roots, the oxygen effect was 

 shown to hold in a variety of material* and was subsequently found to be 

 an important factor not only in chromosome breakage, but in many other 

 forms of radiation damage. 



Two opposed views arose as to the influence of oxygen. Giles and his 

 co-workers* assumed that oxygen increased the initial breakage of chromo- 

 somes, while a number of w^orkers-^- -'^' ''^•'^, took the view that oxygen 

 altered the reunion process rather than the breakage mechanism. Direct 

 experimental evidence for or against these views is difficult to obtain because 

 an unknown number of original breaks restitute the original structure, or 

 reunite to form structures which are indistinguishable from the original. 

 Thus, if the proportion of breaks entering these restitution processes changes, 

 then the apparent breakage also changes. 



Experiments by Giles and Riley'^, Giles*" and Read*^ showed that the 

 oxygen effect in actively metabolizing tissue was only obtained if the oxygen 

 was present at the time of irradiation. From this it seemed likely that oxygen 

 was involved in some direct chemical step which could be assumed between 

 ionization and breakage. It was also known that the effect of oxygen is 

 altered by the type of radiation used. Thoday and Read' showed that the 

 significant oxygen effect obtained with X-rays could not be obtained using 

 a irradiation. Later Giles et al.'^ showed that the oxygen enhancement with 

 neutron irradiation was intermediate between that obtained with X and a 

 irradiation. Gray^^ and Read^' pointed out the similarities between these 

 effects and the known effects of dissolved oxygen in the radio-chemical 

 decomposition of water* ^. Consequently, Gray and Read, elaborating the 

 proposals of Weiss*', have developed a physical-chemical mechanism for 

 radiation damage. On the basis of this mechanism the chromosome 

 aberrations have the following sources : 

 (i) direct ionization of the chromatin thread, 



(2) indirect effect of H and OH radicals formed both in and outside 'target' 

 areas by ionization of water, 



{3) a similar but more widespread indirect effect produced by HO2 and 

 H2O2. 



The direct ionization of the chromosome material, being strictly direct 

 action, would follow target theory expectations. The mean free path of H 

 and OH radicals would be small, as shown by Weiss and Lea, so that 

 although extra target effects are present these are expected to be limited. 

 The contribution by the third source of HO^, HgOg and other powerful 

 oxidants is expected to be considerable as these compounds may have a long 

 mean free path. With radiation giving a high density of ions per unit path 

 length, these substances are formed in watery mediums even in anaerobic 

 conditions. However, H2O2 and HOg are only formed with radiations of 

 low linear ion density such as X and y irradiation when oxygen is present. 



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