NUMBER AND MEANING OF SENSITIVE SITES IN CELLS 375 



radicals themselves. Assuming for the OH radicals a lifetime akin to 

 that in pure water of 10~^ sec, one may calculate that the cross section 

 of a sensitive site might be only of the order of 10~^^ cm^ in the yeast 

 cell. The radius computed from this reminds one more of the size of a 

 bond than the size of a molecule ! However, the cross section may include 

 some unknown factors. There is a possibility that when an intermediate 

 arrives at a sensitive site the probability of its reacting is less than unity. 

 In any case, our hypothetical model of a sensitive site in the yeast cell 

 also gives some information on the mean action radius of H202-like 

 radicals, which should be of the order of 10~* cm. Dimensions of the 

 order of 10~^ cm result for the mean action radius of OH radicals. 



Although the above-outlined ideas represent an attempt to put bio- 

 logical observations on a semiquantitative basis, it is clear that with 

 the availability of better data more detailed and informative physical 

 models could be constructed. The insight into the mechanisms would 

 be materially helped if survival data were available in detail over a wide 

 range of energy loss on several organisms, in particular (a) with certain 

 dissolved substrates, for example oxygen, nitrogen, hydrogen; (6) at 

 different temperatures; (c) with different amounts of water in the cells; 

 (d) at different dose rates; (e) at rates of energy loss higher than those 

 commonty used. The mechanism of biological effects will not be clear 

 until enough of these experiments have been done. Preferably all these 

 data should be available for a number of different organisms. 



Number and Meaning of Sensitive Sites in Cells 



In all the above discussion the existence of a single sensitive site * was 

 assumed without clearly stating the recent evidence for such sites. The 

 advantage of using the diffusion model for fitting survival curves of 

 different kinds is that formally it can be used much like the target theory 

 when interactions of single sites are involved. Since the observed curves 

 were exponential type for the haploid cells and multiple-action type for 

 the diploid cells, the curves were next fitted assuming that one sensitive 

 site existed in the haploid cells (nuclear material damaged anywhere) 

 and two in the diploids. It may be stated definitely that this idea did 

 not fit the data. Next a model, originally proposed by Luria and 



* The name "site" was chosen to distinguish the present form of theory from pre- 

 vious forms. Words like "target" and "hit" were avoided, since the radiation ac- 

 cording to the concepts outhned does not often produce "hits" in "targets." Since 

 we do not know the exact nature of "sites," they are not further specified. A site 

 may correspond to one of several bonds, to a molecule, a gene, a chromosome arm, 

 or some cell component as yet unknown. 



