Discussion 237 



Holmes: I didn't mention the colchicine experiments. We put the 

 colchicine in 3 hours before killing the animal because we had the strong 

 conviction that over a long period it was rather poisonous and decreased 

 the rate of mitosis. That certainly does not happen when it is only in 

 for 3 hours. As regards the induction of radiation injuries, I think I am 

 right in saying that usually one does not get many chromosome breaks 

 if one irradiates cells of Vicia or Tradescantia early in interphase before 

 DNA synthesis, but that was not true in the case of regenerating liver. 

 Many breaks were found when irradiation with 450 r was given before 

 the beginning of the period of DNA synthesis, i.e. at 12 hours after 

 hepatectomy and 13 or 15 hours before mitosis would be expected. 

 This would normally be described as irradiation during interphase. 



Roller : Yes, that was the case. 



Howard: How long was the colchicine present? 



Holmes: Only the 3 hours. The ^^p was also injected 3 hours before 

 killing; we tried to collect all the mitoses that happened in that period. 

 There are no colchicine counts shown in Fig. 1. 



Lajtha: This was surgical hepatectomy? 



Holmes: Yes, ours was surgical hepatectomy. 



Lajtha: The difference between your results and those of Dr. Kelly 

 may be due to the different means of producing hepatectomy. She pro- 

 duces it by chemical means, and that does not destroy the nuclei com- 

 pletely. Perhaps she has an experimental condition similar to that which 

 Dr. Forssberg has, with some DNA present in dying cells which may 

 serve as a pool for the regenerating cells. 



Holmes: That is a very interesting suggestion. 



Gray: Do I understand correctly that the wave of mitosis is not delayed 

 if you irradiate just before hepatectomy, but that it is if you irradiate 

 after hepatectomy? 



Holmes: No, if you irradiate just before there is just as much delay 

 as if you irradiate in the "sensitive" period. If you irradiate it 24 hours 

 before hepatectomy there is some recovery. The rate of DNA synthesis 

 is low and some chromosome breakage is seen, but some mitoses do 

 appear at 27 hours. 



de Hevesy: Do you know the actual growth rate, in the course of 

 1| hours, of those tumours in your last table? What percentage would 

 that be ? They can't grow without formation of DNA. 



Holmes: I do not know. 



de Hevesy: The ratio of the specific activity of DNAP at the end of 

 your experiment and the mean value of the specific activity of cellular 

 orthophosphate P during your experiment is supposed to give the per- 

 centage of DNA molecules formed in the course of 1| hours. If you 

 follow up the growth of your turnover for a longer time and extrapolate 

 from these data the amount of DNA formed during 1^ hours you will 

 presumably find half of the value supplied by the radioactive data only. 

 We and others interpreted such findings in the old days as indicating 

 that with the formation of two new DNA goes hand in hand the dis- 

 appearance of one. Recent work carried out by you and others indicate 

 however that no appreciable amount of DNA molecules disappears in 



