BIOLOGICAL EFFECTS OF RADIATION 117 



inapplicability of one or more of its assumptions. From an analysis of 

 the failures, one should then learn even more about the nature of the 

 biological entities and about radiobiology. 



The target theory may be applied, in part, to studies utilizing ultra- 

 violet radiation, for these are also energetic enough to ionize atoms. The 

 restriction stems from the fact that although the probability of an ultra- 

 violet photon's being absorbed by matter is proportional to the electron 

 density (and thus to the volume of the matter), there is no theoretical 

 connection between the radiosensitive volume and the volume of any 

 substantial portion of the organism involved. The photon's absorbing 

 element is really the electron taking part in a bond, and its volume is not 

 greatly dissimilar to that of the bond itself. Therefore, although a vol- 

 ume can be computed, it doesn't teach us anything. Furthermore, if we 

 break our substance into pieces, the absorption probabilities of the pieces 

 do not total to the probability of the united substance. In sum, then, 

 one cannot compute a volume from ultraviolet inactivation data. How- 

 ever, since the action appears to be due to a single ionization process, 

 the kind of target analysis we have presented may still be applied to 

 deduce the number of targets. Also, from the asymptotic slopes of the 

 inactivation curves, we can say whether the targets are the same or not. 



BIOLOGICAL EFFECTS OF RADIATION 



Up to this point, we have been analyzing the physical action of 

 radiations on biological systems. There remains the subject of the bio- 

 logical effects. We shall discuss the following examples of biological 

 effects: mutation, ultraviolet-light inactivation and reactivation. 



1 . Mutation 



Since the initial demonstration by Muller that x-rays can induce 

 mutations, an extensive body of literature has developed on this topic. 

 Aside from the straightforward use of radiations to induce mutations 

 for the sake of obtaining mutant organisms, the kinds of mutations in- 

 duced permit deductions as to the nature of mutation itself. X-ray 

 mutagenesis has been found to include chromosome breaks, transloca- 

 tions, and other cytological effects which offer strong evidence that the 

 x-ray simply breaks the chromosome. Ultraviolet light mutagenesis has 

 recently been given an enormous stimulus by the finding that physiolog- 

 ical doses effect the formation of dimers of one of the four bases making 

 up nucleic acids. In such a situation, as the nucleic acid is copied during 

 its replication, the copying mechanism is very likely to err when it 

 reaches the dimer, thereby producing a mutation. Such a mechanism is 

 restricted to the locations having two identical bases in series along the 

 nucleic acid chain. It is not known whether x-rays can also act this 



