378 ENERGY LOSS AND BIOLOGICAL EFFECTS 



vated, the cell will not form macrocolonies." Notice that ''site" has 

 very little to do with target; ions do not have to hit the site. 



A pair of corresponding sites has to be inactivated out of about 20 

 pairs of sites before inhibition of colony growth occurs in diploid yeast 

 cells. When development of the yeast cells is observed under the micro- 

 scope, it is found that inhibition of cell division occurs only after a 

 number of cell divisions have taken place. Microcolonies develop; 

 inhibited haploid cells divide usually only once after irradiation; in- 

 hibited diploids may divide several times before growth of the micro- 

 colony ceases. The rate of growth of inhibited cells and also of a few 

 cells which will eventually grow into macrocolonies is slowed down con- 

 siderably. It is thought that there are important cytoplasmic effects 

 along with the damage to the essential sites. In addition there may be 

 non-lethal damage to a number of genes in the cell. With only the hap- 

 loid and diploid strains available, the above-advanced theory of inhibi- 

 tion of cell division seemed plausible but not conclusive. More evidence 

 was added in favor of this theory by Mortimer (37), who, following a 

 method given by Subramaniom (38), isolated some apparently multi- 

 ploid yeast cells. Such cells have been obtained so far by two different 

 methods: treatment with acenaphthene and isolation from preirradiated 

 diploid yeasts. These appeared to be morphologically like the tetraploid 

 yeasts of Subramaniom, although to date we have not succeeded in ob- 

 serving the chromosomes in the microscope or in obtaining genetic proof 

 of ploidy. The inhibition of cell division in these cells of higher ploidy 

 follows reasonably well what is expected for tetraploid yeasts on the basis 

 of Eq. 10. In fact, the experimental data seem rather amazing (Fig. 12) : 

 the tetraploid yeast cells need 140,000 rep units of x-rays for 50 per cent 

 inhibition of cell division, which is 4 times the dose required by the 

 diploid cells and 20 times the dose required by the haploid cells. Morpho- 

 logically, and as far as general viability is concerned, there seems very 

 little difference in the yeast cells of different ploidy. These experiments 

 indicate that at least in yeast cells radiation injury of the genetic 

 mechanism, although mediated by the cytoplasm, is a much more 

 important factor than extragenic radiation injury. 



What is the nature of the n essential sites of radiation injury? Al- 

 though no direct evidence is available, it is plausible to assume that the 

 sites are part of the chromosomes or genes. The damage might well be 

 chromosome break or gene mutation. Further work is needed to identify 

 the nature of the sites and to determine whether or not there is the same 

 probability for inactivating each of the sites. One of the requirements 

 for a satisfactory theory of radiation effects is that the explanation 

 offered should hold when the experiments are repeated under different 



