NUMBER AND MEANING OF SENSITIVE SITES IN CELLS 377 



the survival curves are not very accurate, as in the present experiment, 

 the value n may be obtained. At the present time we believe that n is 

 about 20 for ^S. cerevisiae. Thus, if the model used were true, one might 



100 



:-5 80 



60 



Z 40 



" 20 







20 



40 



60 

 Dose, krep 



80 



100 



120 



Fig. 10. Theoretical survival curves for haploid and diploid yeast cells adjusted to 



fit the experimental curve for haploids. The possible diploid survival curves are 



plotted for n = 1, 2, etc., up to n = 64. 



make the following statement: "Haploid yeast cells under the experi- 

 mental conditions applied exhibited approximately 20 radiation- 

 sensitive sites essential to cell division. If any one of the sites is inacti- 







10 



15 



20 



25 



30 



35 



Fig. 11. The ratio of doses needed for 50 per cent inhibition of cell division, 

 D50 diploid/Dso haploid, is plotted here as a function of the number of essential 

 sites n. As it may be seen from the curve, this ratio is quite sensitive for the 



number of essential sites. 



