POST-IRRADIATION AND BONE-MARROW COUNT 325 



may happen is that, before a cell undergoes repair, certain of these nucleotides may leak 

 into the blood more rapidly than others so that the cell is confronted by a sample of the 

 various nucleotides which is quite different from the normal distribution of nucleotides. 

 Now if the samphng is too abnormal so that no regeneration is possible, maybe these 

 nucleotides can be used by another adjacent cell. M. Errera, for example, proposed a 

 long time ago, that the abnormal proportion of various nucleotides within the cell after 

 irradiation may actually be the biochemical mechanism of mutation. 

 DRASIL: I am afraid that the cells in wliich the DNA is hydrolysed must die and that we 

 can help only those cells which were only damaged reversibly by radiation. This is one 

 thing; the products of Itydrolysis of DNA, the single nucleotides, may be quite different 

 from those deoxynucleotides needed for synthesis. 



UPTON: All of us are familiar with the healing of chromosomes, cliromosome breakage and 

 restitution. Is there any evidence that these phenomena involve depolymerization of 

 nucleic acids and then re -polymerization or are they explicable in simpler chemical 

 terms? 



COTTIER: The only thing I can say about this is that after thymidine labelling the label 

 stays m the chromosomes and I don't thuik that anybody has ever seen leakage of label 

 into the cytoplasm. We also examined tissue that had been irradiated after labeUing 

 with thymidine and I could never see any evidence of leakage of label from the nucleus to 

 the cytoplasm except in dead cells. 



