364 



ENERGY LOSS AND BIOLOGICAL EFFECTS 



taught us that water ions themselves last for very short time intervals 

 after their production. Dissociation occurs, giving rise to H and OH 

 radicals; these in turn may combine with each other and with cellular 

 constituents. Dale attempted to explain the general shapes of dose- 

 effect curves as well. He showed that monomolecular reactions lead to 

 exponential dose-effect relationships, curves similar to those one obtains 

 in the "one-hit hypothesis"; further, he has shown that, if there are two 

 substances competing for ionization products, the dose-effect relation- 

 ship becomes very similar to the multiple-hit-type functions of the 

 target theory. 



Comparison of Two Theories in the Light of 

 Experiments on Yeast Cells 



In the experiments described here the dose dependence of the survival 

 of haploid colonies was compared to that of diploid colonies. If the 

 indirect-action theory were true, one would expect the two survival 



100 



80 



60 



40 



20 







10 



20 



30 40 

 Krep 



50 



60 



70 



Fig. 4. Survival of haploid yeast cells as a function of dose when x-raj^s and alpha 



particles are used. The curves indicate the percentage of cells that divide more than 



once after irradiation. Single-hit-type curves. 



curves to be qualitatively the same. Since the biochemical constitutions 

 of the haploid and diploid cells are assumed to be similar, the survival 

 curve of both colonies might show monomolecular reactions, or both 

 might exhibit the presence of some protective substances. Actually the 

 haploid cells showed the simple exponential type of survival vs. dose 

 relationship (monomolecular reaction) in a wide range of REL. The 

 survival curve of diploid cells showed some kind of "protection" effect 



