NUMBER AND MEANING OF SENSITIVE SITES IN CELLS 381 



preirradiated diploid colony may be also calculated (Fig. 15). For 6 

 months experiments were carried out on diploid colonies grown from 

 preirradiated single diploid cells. A number of colonies were found to 

 show defective survival curves. These colonies are demonstrated easily 

 because the initial slope of their radiation-survival curves differs from 

 zero, which is the slope of normal diploid colonies. An example is shown 



25 



20 



Q,'- 



15 



10 



Pr(n) = ri'ne 



2{n-r)ctD,-0iD 



Cr') = 



_ n(n-l)- 



■(n-r+\) 







X 



W/////M 



\:--^:."\ ^ 22 krep 86% survival 

 1%^ -^ 47 krep 58% survival 

 i ' :■ "1 ^ 70 krep 32% survival 



Assume n = 16 

 LD50 = 53 krep 



^^^w.^ I 



Ix 



wy///////// 



r 



4 5 6 



Number of defects, r 



10 



Fig. 15. Theoretical distribution of diploid yeast cells with unpaired defects pro- 

 duced after single irradiation of normal diploids. 



in Fig. 16. However, none of these colonies retained their defective 

 properties for more than 5 weeks. Some mechanism appears to be 

 present which allows regeneration of normal diploid cells, which then 

 outgrow in numbers their recessive sisters. Nevertheless, there is hope 

 that stable and recessively defective diploid yeast colonies will be avail- 

 able for future study. It is interesting that preirradiated haploid cells 

 show the same survival curves when irradiated again as normal 

 haploids, a finding that strengthens the explanation given. In other 

 words, for haploid cells, inactivation of a single site leads to complete 

 inhibition of cell division. At the present time it is not known whether 

 the probability of inactivating each of the n essential sites is the same or 

 not. It is likely that the probability will be different for each site, the 

 overall effect being a small deviation in the shape of the diploid survival 

 curve. The deviations in the shape will also show up in the cells with 



