114 



been tampered with. But since the cell did not need this mechanism until it was 

 going to divide, no effect was seen until it began to do so. The fact is in these 

 cells growth goes on unchanged, there is synthesis of the ribose nucleic acid and 

 also when you reactivate with light the division rate is resumed. 



CURTIS: Whose work is this? 



KAMEN: Kellner (14). 



PATT: Did he observe an early effect on DNA? 



KAMEN: That is the point I am coming to. Immediately after the radiation, 

 he found the DNA synthesis stopped. Upon reactivation, DNA synthesis was re- 

 sumed. This is a very interesting piece of work because it appears to be the 

 first time somebody has found a process which is interfered with almost imme- 

 diately. That holds out some hope. It came about only because of the use of the 

 reactivation approach. 



This may be another example of what Dr. Burton means by a new approach. 

 We have to wait for new techniques. 



PATT: Was there any change in the absolute amount of DNA? 



KAMEN: No. There is actually an increase in the amount of DNA in the 

 cell. These are growing cells and there is synthesis. 



MAZIA: It is a convenient system. Because the turnover is so low P^ 2 in- 

 corporation gives a pretty reliable measure of synthesis. 



PATT: Hevesy (15) has referred to this type of mechanism for some time. 



KAMEN: But it is the first time that anybody has gone back to the event 

 rather close to the cessation of radiation. 



PATT: An interesting point in this regard is the fact that rather early ef- 

 fects on DNA turnover have been detected in sites distant from an irradiated 

 area. 



KAMEN: The trouble is, of course (and this is where Dr. Pollard's question 

 comes up, which was what is the function of the nucleic acid in general) that we 

 don't know what they are doing in synthesis. With regard to measurements of 

 nucleic acids one would like to know if it is the nucleic acids that are affected or 

 the protein that is affected, and when we know something about the structure of 

 the nucleic acids and more about the protein structure, we should know a great 

 deal more about those things. There is no objection to indulging in speculation 

 about what happens to such structures when irradiated. 



To continue with some other points. There are experiments on irradiating 

 together and separately an enzyme plus its nucleotide prosthetic group (and most 

 enzymes have prosthetic groups). Very often you can take them apart and put 

 them together again. 



You can take the prosthetic group and irradiate it separately -- likewise with 

 the apoenzyme -- and then recombine them. The total radiation effect which is 

 observed is additive, that is, the per cents of inactivation for each part add up 

 to what is observed for the two irradiated units recombined. 



