Discussion 211 



Holmes: Could it be that you have got enough adenine present and that 

 the cells are simply using that ; some cells use it preferentially, and don't 

 synthesize de novol 



Davidson: We consider this unlikely since the pool of acid-soluble 

 adenine compounds in the ascites cells seems to be small. Indeed we are 

 inclined to regard these cells as parasites on the purine-synthesizing 

 mechanisms of their hosts. They can, of course, utilize intact purines ; 

 that does happen. 



Lajtha: It must happen, because we gave some aminopterin in low 

 concentrations to cultures, and this prevents the i*C-formate incorpora- 

 tion into thymine, but it did not prevent the i*C-adenine incorporation 

 into DNA. Now since we were not prepared to believe that these cells 

 synthesize a thymine-less DNA, we thought that there must be a pool of 

 thymine and that there must be a pool of adenine as well, so that if we 

 prevent the incorporation of labelled formate the cells can still use their 

 preformed pool substances. 



Davidson: There is one interesting point here, and that is that in the 

 ascites cells which normally do not incorporate formate in vitro to any 

 appreciable extent into the DNA purines, the addition of a particle-free 

 saline extract of liver cytoplasm will very markedly stimulate the incor- 

 poration of formate into the purines of both RNA and DNA. 



Howard: We all agree that the usefulness of the autoradiographic 

 method depends on biochemical analysis and biochemical identification 

 of the compounds, and this is not a very easy matter on which organic 

 chemists can at once agree. On the other hand, the autoradiographic 

 method is the only tracer method of looking at individual cells and this 

 seems to be a very important thing to do. Therefore, the autoradio- 

 grapher is in the biochemist's hands for advice on the identity of the 

 compound. 



Alexander: Is there sufficient data to make a clear distinction between 

 interphase cell death and cell death following division? Can the possi- 

 bility be completely excluded that cell death occurs on average at a time 

 after irradiation which depends on the size of the dose ? 



Howard: We can be fairly certain that there are these two kinds of cell 

 death. 



Lajtha: With regard to Prof. Brachet's remark about the possibility 

 of exchange, we calculated the number of molecules getting into the cell, 

 into DNA, and both with i'*C-adenine and with i*C-formate we got 

 identical numbers as far as the technique allows: of the order of 20 

 million per cell DNA. Since this is valid both for thymine and for 

 adenine, if that had been exchange that would imply exchange of 20 

 million adenine-thymine pairs, and I find it very difficult to believe that 

 such an extent of exchange can happen if we accept the helical structure 

 of DNA. 



