460 



RADIATION BIOLOGY 



photoreactivatioii tend to striiight lines with ditTerent slopes. The slope 

 is an important (luantitative datum. In fact, if a survival curve of an 

 irradiated population is a straight line, its slope determines the cross 

 section for the radiation of the individual elements. In multiple-hit 

 curves the slope of the straight line to which the curve tends at high doses 

 of the radiation has the same meaning; in an unhomogeneous population 

 the final slope measures the cross section of the individuals of the class 



ULTRAVIOLET DOSE , seC 

 100 200 300 



ULTRAVIOLET DOSE , seC 



400 



10 



20 30 40 50 



60 



10" 



; io'2 



10" 



o 



Q 



> 



lOO 

 50 





o 

 o 



o 



^ 30 r 



20 



10 >- 



I 

 o 



100 200 300 400 



DARK- ULTRAVIOLET DOSE.sec 



(a) 



60 



10 20 30 40 50 



OARK-ULTRAVIOLE T 



DOSE , sec 



(b) 



Fig. 12-1. Survival curves in darkness and after maximum photoreactivation for (a) 

 phage Tl and (b) phage T2. Curves I are for darkness; II, after maximum photoreac- 

 tivation; III, curve giving the ultraviolet dose corresponding to a given survival after 

 photoreactivation versus the dose corresponding to the same survival in darkness (see 

 Sect. 5-3). For survival curves, log survival (left-hand scale) is plotted versus the 

 ultraviolet dose in seconds of exposure (upper scale). For lower curves, light-ultra- 

 violet dose in seconds of exposure (right-hand scale) is plotted versus dark-ultraviolet 

 dose (bottom scale). 



with least cross section. The fact that in phages the survival curves in 

 darkness and after maximum photoreactivation have different final slopes 

 can be taken as a demonstration that the cross section of the particles to 

 ultraviolet is reduced after photoreactivation. This means that the 

 damage produced by ultraviolet in bacteriophages can be divided into two 

 classes, one completely photoreactivable and the other nonphotoreac- 

 tivable, with a constant ratio, independent of the ultraviolet dose. Exist- 

 ence of these two classes of ultraviolet damages can be explained under 

 two different assumptions : either ultraviolet light produces two different 

 types of chemical effects, of which only one is photoreactivable; or only 



