210 Discussion 



that idea. Some cells may be producing DNA as a sort of secretion 

 product, and in that case one might find more DNA or perhaps DNA 

 of a different kind, or a different degree of polymerization, in such cells ; 

 but these would also be exceptional cases. 



Bracket: It appears that some workers do not quite agree with this 

 view, which is held by most American workers. Dr. Fautrez came to the 

 conclusion that, with the same apparatus, he can get different results 

 under different physiological conditions. . 



Howard: I know of Fautrez' work, and I agree entirely that one should 

 not be dogmatic, but still I think it is up to him to prove his point, 

 because at least in some of his work insufficient allowance has been made 

 for synthesis due to preparation for mitosis. One has to have a pretty 

 complete knowledge of the changes in cell population that are going on 

 over a period of time to exclude this reason for different DNA values, 

 and this has not been sufficiently allowed for. 



Davidson: I think it is true that in those cases where there are devia- 

 tions from what one might call the Boivin-Vendrely rule, if I may use 

 the term, the cells have been put under quite abnormal conditions, 

 and that if you stick to physiological conditions the rule does follow 

 fairly well. It is impossible to generalize completely, but on the whole I 

 think the amount of DNA per chromosome set remains unchanged under 

 ordinary physiological conditions. 



There is one point I would like to make in relation to Prof. Brachet's 

 earlier remark about ^^P incorporation into DNA. We have recently 

 been doing a lot of work on incorporation of various precursors into 

 ascites cells in vitro, and the situation there is that under the conditions 

 employed there is excellent incorporation of ^^p jnto DNA, good 

 incorporation of [8-^*C]adenine, but next to no incorporation of labelled 

 formate or labelled glycine; this suggests that purine synthesis just does 

 not occur, and presumably DNA synthesis does not occur to any 

 appreciable extent either, although it is very difficult to measure the 

 total amount of DNA because the increase one would expect would be so 

 small as to be within the experimental error of the estimation. There is 

 no doubt whatever that ^^p jg in fact incorporated into the DNA, 

 because we have degraded the DNA to the individual deoxy nucleotides 

 and separated them and found incorporation into each individual 

 nucleotide, just as into the whole DNA. 



Spiegelman: Some very careful experiments have been done recently 

 by Siminovitch, using cultures of bacteria as well as tissue, in an attempt 

 to detect such a turnover, and he finds none. 



Gray: Prof. Davidson, I infer from what you say that you thought 

 that there was no cell growth going on in your in vitro preparations. Is 

 this the point of your remark ? 



Davidson: It would appear that there is no de novo purine synthesis 

 going on, as indicated by lack of incorporation of formate and lack of 

 incorporation of glycine. There is good incorporation of formate into the 

 thymine of the DNA, as Totter found with marrow cells in vitro (Totter, 

 J. R. (1955), J. Amer. chem. Soc, 76, 2196). Incorporation into the 

 methyl group of thymine is excellent. 



