Discussion 209 



Alper: It is known from Stapleton's work that the sensitivity of 

 bacteria is quite different if you irradiate them in the stationary phase 

 before they have started synthesizing anything at all, and just at the 

 end of that when they are about to go into the log-phase. It certainly is 

 tempting to feel that somehow this lack of sensitivity is due to the fact 

 that those about to enter the log-phase have already got their DNA 

 synthesized. 



Hollaender : However, the story is different with u.v., where you have 

 the opposite effect to that obtained with X-rays. You have a very high 

 sensitivity immediately before they go into the log-phase. This may be a 

 question of absorption which has never been determined. 



Alper: I have found that if you take bacteria in the stationary phase 

 and irradiate them, you get prolongation of the lag-phase, but if you 

 irradiate the bacteria which are just about to go into log-phase and plot 

 the growth curve after that, you get an increased lag-phase, and then 

 you get the catching-up effect which I mentioned and very much less 

 cell death. 



Gale: May I ask if those cells are really not growing or are they just 

 producing morphologically odd forms ? 



Alper: The experiments I have just mentioned were all done on viable 

 counts. But I have also been doing some morphological work and get 

 quite distinct dose-dependence curves, whether I am looking at the 

 added lag, at the number of long forms produced, or the number of long 

 forms that will go on and produce colonies. 



Bracket: Dr. Howard, are you completely satisfied that ^^p^ or any 

 other precursor you are using in the autographic method, is really an 

 indicator of the time of DNA synthesis ? Can you rule out any turnover 

 of DNA? There is also the problem of the constancy of the DNA 

 content of the nucleus ; I am quite willing to think that it is approxi- 

 mately constant. I am willing, also, to think that DNA is relatively 

 stable, but I am not absolutely convinced that DNA is completely inert, 

 and that it is always entirely constant. 



Howard: In answer to your first question, I think that from autoradio- 

 graphic work which Dr. Pelc and I did with the bean root, the period 

 of uptake of ^^p into DNA is reasonably in agreement with the period 

 during which the DNA is increased. But as regards the adenine labelling 

 in the ascites tumour the situation is much less clear, and there is a good 

 possibility there that the time at which the DNA is labelled with 

 adenine does not coincide with what one can observe biochemically, i.e. 

 an increased amount of DNA in the cell. This is still an open question, 

 because the biochemical results seem to be in serious conflict with each 

 other. 



With regard to the constancy of DNA per chromosome set (I think we 

 should say that, rather than per nucleus), there seem to be a few 

 exceptional cases in which too much or too little DNA is found, to be 

 consistent with this theory. But the exceptions are rather few, and I 

 feel fairly satisfied that it is a general rule that the chromosome carries 

 an amount of chromosomal DNA which is fixed for that chromosome. 

 It would take a good deal more evidence than now exists to overthrow 



