376 ENERGY LOSS AND BIOLOGICAL EFFECTS 



Dulbecco (36) for ultraviolet-ray-induced effects, was tried. This model 

 was used when the lethality of ultraviolet rays was tested on bacterio- 

 phage, attached singly or doubly to bacteria. Applying the model to 

 the haploid yeast cell, we may calculate the number of cells N which 

 survive and form large colonies, when initially A^o are irradiated by 

 dose D. 



N = Noe-""^ (9) 



where n — number of independent sites essential to cell division and 

 inactivated by a single intermediate molecule or radical. 

 a = ^ap/w = constant (see Eq. 3). 

 D = dose. 



Haploid yeast SC7 and SC8 



Normal , ^^ , 



N = Noe 



Diploid yeast SC6 



nhibited 

 Normal 



aD -2oiD\n 



iV = iVo(2e""^-c ^'^") 



A''= survivors 

 D = dose 



a = constant 



n = number of compartments 



Fig. 9. Schematic presentation of model for inhibition of haploid and diploid yeast 

 cells by radiation. It is assumed that the haploid cell has n independent sites es- 

 sential to cell division. If any one of these sites is inhibited as shown by the black 

 spot in the haploid cell in the upper right part of the figure, the cells will stop dividing. 

 In the diploid yeast cells the number of essential sites is doubled; here a pair of es- 

 sential sites has to be inhibited before cell division stops. 



For similar cells of a ploidy m 



N = No[l - (1 - e-^^rr (10) 



which for the diploid cell becomes 



For graphic explanation see Fig. 9. With the survival data (Figs. 4 and 

 5) it was possible to plot the theoretical haploid-diploid survival curves 

 as shown in Fig. 10 for different values of n. One may notice that the 

 shape of the curve and the D^o are sensitive functions of n. The theo- 

 retical ratio of Diploid Z)5o/Haploid Dso is plotted in Fig. 11. Even if 



