64 



SHERMAN: This is counts per minute per milligram dry weight. The 

 earliest point we have is at 15 minutes, and we have carried these out to about 

 three hours. 



COHN: This is not specific activity? 



It could be specific activity only if the phosphorus content had a linear 

 relationship to milligrams dry weight. 



SHERMAN: That is right. If it does not, then you might guess that 

 the cells are taking up P^^ preferentially. 



COHN: No, I would think that possibly you could have here an enor- 

 mous net increase in the nucleic acid, a doubling or tripling of the actual milli- 

 gram content of the culture, let us say. If you have an increase in total phos- 

 phorus you will naturally have an increase in P-^^ if you select the medium that 

 way. These are irradiated in a phosphate buffer though. 



SHERMAN: These are irradiated in a phosphate buffer and these cells 

 are not starved cells. There was nothing wrong with them before irradiation as 

 far as we knew. 



COHN: You see it is important. The only point that I am trying to 

 make is that it is important to consider specific activities not only in phosphorus 

 but in terms of dry weight and in terms of the cell itself and in the degree of 

 polyploidy if you are talking about DNA. Even using DNA as the criterion for the 

 number of cells, you can run into trouble unless you know the degree of poly- 

 ploidy also at the time of measurement. The mg. dry weight, I think, is not a 

 very good measurement. One can duplicate this type of thing by the loss of some 

 nonspecific weight constituent of the irradiated cells. 



SHERMAN: Yes, I agree with you, but I wonder if you would have this 

 kind of change in 1 5 minutes. 



COHN: My point may be theoretical, but there is a tendency to bring 

 up experiments that we never heard about, and I do not think that all of them 

 have paid too much attention to the denominator. 



POTTER: I would hazard a guess that if you put it in the terms that 

 Dr. Cohn wants, the effect might be even more striking; that these irradiated 

 cells are in effect phosphorus-starved cells and that, in the course of irradiation 

 they have lost ground which they are now gaining back, whereas the normal cells 

 are not turning over as fast. 



COHN: If what you say were correct this would bring the two curves 

 together essentially, because what you are saying is that the irradiated cell has 

 to gain phosphorus back to make it up. If it does, it will also gain P-^^ and you 

 really won't have an effect. 



CHARGAFF: You don't know whether the cell lost phosphorus during 

 irradiation'? 



^2 SHERMAN: I don't know that, but I do know that there was no loss of 



P at the end of the 3-hour period when the cells were washed and suspended in 

 p-'^-free buffer. 



CHARGAFF: It would be important to have a comparison analysis right 



