NUMBER AND MEANING OF SENSITIVE SITES IN CELLS 379 



environmental conditions. Already data are available which indicate 

 at least limited invariance of the relative shape of survival curves. 

 There is a set of experiments available at different rates of energy loss. 

 The relative shapes of haploid and diploid survival curves in all these 

 experiments are substantially the same even though the relative bio- 

 logical effectiveness, that is the amount of dose required, changes as a 

 function of REL. Further, these organisms were tested with x-rays 



Krep 



Fig. 12. Recently'a tetraploid yeast colony was isolated and tested for survival 

 with x-rays. The respective haploid, diploid, and polyploid survival curves are 

 shown for cells which divided more than once. The tetraploid curve comes reasonably 

 near the theoretical relationship based on the assumption that a quadruplet of es- 

 sential sites out of about 16 quadruplets of sites has to be inactivated before the cell 

 division of tetraploid cells is inhibited. The dose as given in this graph is slightly 

 different from those in Figs. 4 and 5. Data in this graph were counted after 2 days' 

 incubation; Figs. 4 and 5 refer to data counted after 3 days' incubation. 



when they had dissolved helium, air, or oxygen gas present in their 

 cytoplasm. Much less dose was required in oxygen to produce inhibition 

 than in helium, but the relative shapes of survival curves, thus the 

 apparent number of essential sites, remained the same in each experi- 

 ment. 



The shape of the survival curves having been established, further 

 methods were sought in an effort to test the theory of inactivation. The 

 study of cells which were damaged but which survived the radiation 

 offers a convenient step in this process. In order to explain the mecha- 

 nism of action of radiation it was assumed that diploid cells with un- 

 paired defects survive. These cells are damaged but viable. If the 

 damage is genetic, as assumed above, it is likely to be inherited in a 

 vegetative colony as a recessive defect. Colonies grown from preirradi- 



