GENERAL DISCUSSION 337 



onion rootlets. In Urosophila under controlled conditions 0-41 ±0-008 

 per cent mutations are obtained. Under the action of small concentra- 

 tions of streptomycin on ripe sperm, 0-10 ±0-02 per cent mutations 

 occurred. In the onion rootlets in controlled conditions there were 

 1-83 ±0-3 per cent chromosome arrangements and under the action of 

 streptomycin 0-17 ± 0-17 per cent. Thus, in a natural mutation process 

 the action of streptomycin is to repair. In this case there occurred, as 

 it were, a "healing" of the hereditary substance which is accomplished 

 at the molecular level. 



In conclusion, I must say that even taking into account the repair 

 phenomena the following basic principles hold even for the case of 

 genetic effects : 



1. The absence of a threshold radiation dose. 



2. The existence of a direct dependence of the frequency of the 

 mutations upon the dose, and, 



3. The cumulative effect of small radiation doses. 



All this leaves the stable position that the influence of any, including 

 all, small radiation doses is for a human being genetically harmful. 



hollaender: Russel has data that show that true gene mutations 

 occur in the "S" locus. I would like to suggest that there is one further 

 consideration. Russel has never claimed that he can rej)air gene muta- 

 tions. But he has dealt with a process which leads to the appearance of 

 mutations. The cytological work connected with these problems is 

 now being carried out. In the next six months the opportunity will 

 occur of reporting the results. 



We can see from Dubinin's contribution that he is also working on 

 this problem, but he is dealing with another type of organism and the 

 further development of this work will be of very great importance. 



errera: I would like to say a few words concerning cytoplasmic 

 repair. 



Bracket has shown that cytoplasm in the absence of a nucleus is 

 capable of being restored, but only for a very limited period of time. 

 Consequently, damage to the nucleus is important also for the restora- 

 tion of cytoplasmic lesions in the cytoplasm. This is very well demon- 

 strated by the ageing of irradiated cells. 



BACQ : IV. Chromosome breaks are not only the result of direct radi- 

 ation effects. They are the result also of secondary effects and can be 

 modified by the metabolic activity of the cell. 



