DNA Synthesis 151 



particularly in liver, there are different classes of nuclei, some of which 

 contain twice as much DNA as the usual amount, others which contain 

 four times as much, and others which contain eight times as much. 

 This has come out of recent work by Swift, and by Leuchtenberger and 

 the Vendrelys. Therefore the synthesis of DNA must be related not 

 only to mitosis but also to endomitosis, or whatever the process is 

 which gives rise to this doubling of DNA content, presumably associated 

 with doubling of the chromosome number. Other organs such as the 

 kidney show far less markedly this appearance of high DNA content. 

 This might explain some of the difficulties associated with work on 

 DNA synthesis in so-called non-mitotic tissue or tissues in which the 

 visible mitotic rate is very low. 



McFarlane: I am particularly interested to learn that both investi- 

 gators find that DNA synthesis is depressed after radiation, but can 

 find no similar effect on the synthesis of proteins associated with the 

 nucleic acid. I do not know of any figures for the histones of the nucleus 

 either in resting or dividing cells, but certainly in normal tissues the 

 turnover period of the DNA is of the order of days, while that of the 

 cytoplasmic proteins is of the order of hours. May this not explain 

 why you find no radiation effect on intracellular protein metabolism — 

 namely that the effect is evident only in a much shorter time interval 

 than your experiments cover? 



Holmes: I think that is so. I think it is obliterated by the tremen- 

 douse amount of turnover that goes on, quite apart from any new 

 synthesis. Professor Hevesy suggested that when I first told him about 

 it. On the other hand, if this turnover is going on just as fast as ever 

 after X-rays, I should think it must produce curious results in the cell. 

 I have been dealing with the histones, having been forced to discard 

 the other chromosome proteins, because you can't do both, and I think 

 almost certainly Miss Howard has been dealing with the other proteins 

 of the chromosome and washing out the histones; thus we seem to 

 have both proteins represented in the work. But I do think it is possible 

 that it is just such a big general turnover that any extra bit due to the 

 formation of a new nucleus wouldn't necessarily show, but that the 

 X-rays don't slow up that rate of turnover. 



Pelc: But there are differences in our results which I don't think can 

 be explained by turnover. After 20 or 24 hours many of the elongated 

 cells show no autoradiographs of the nucleus. The amount of ^^S in 

 the protein of the differentiated cells should indicate the turnover rate, 

 since ^ss was available to these cells. The only thing which might make 

 any difference there is the change of permeability of the membrane, a 

 point about which we know very little. 



