79 



just mentioning one and that is that we don't have the DNA curve to relate to the 

 RNA and protein changes. 



MAZLA.: As I said, we are facing technical difficulties in measuring 

 DNA formation in the ameba. We are trying to measure it photometrically, and 

 the thing that is holding us up would not interest the group here; it is the fact 

 that in Feulgen staining the DNA is so coarsely distributed as to make the situa- 

 tion very unfavorable for spectrophotometric operations. 



In any case, answering your last question, I am proposing here a DNA 

 unit that has something to do with the production of RNA in a cell that isn't divid- 

 ing and that is undergoing duplication in a cell that is getting ready to divide. 



To block division, one could conceivably block the duplication of this 

 parent molecule without necessarily affecting its ability to produce RNA. Let's 

 put it this way. We have a DNA unit of a certain kind and only one such unit is 

 needed to produce RNA, but you have to have two of them before the cell can 

 divide. So you can imagine situations where irradiation will block division with- 

 out blocking RNA synthesis or others where irradiation will block RNA synthe- 

 sis. It could even be a quantitative difference. It all goes back to elementary 

 biological considerations that do not depend on any of our chemical assumptions. 

 The genetic material of the cell (which may be DNA) clearly has two different 

 functions. One is to serve the cell that it is living in. The other is to double 

 itself so that both progeny after division will have as much of it as the mother 

 cell. 



KAMEN: The DNA goes up to a constant level just before division. 

 You have doubled the thing before you start. 



MAZIA: You double the DNA before division. If you observe a cell in 

 a random population, it may have a single dose of DNA ( actually the diploid 

 amount ) if the cell is young, a double ( tetraploid ) dose in a cell that has com- 

 pleted its growth and is going to divide, or intermediate values in a cell that is 

 somewhere between divisions. This has been established as a fact by the cyto- 

 chemists. Furthermore, the cytochemists find that cells that are unlikely ever 

 to divide again ( as in highly differentiated tissues such as kidney ) almost al- 

 ways have the single dose. They make no DNA after their last division. On a 

 statistical basis, the majority of cells in an actively dividing population will have 

 the intermediate amount of DNA. 



SHERMAN: Or more. 



MAZIA: That's right. You will find a good many cells approaching 

 division and therefore approaching the double dose of DNA. 



BENNETT: In some tissues, I think, irradiation does not stop cell 

 division after a certain time. 



MAZIA: Dr. Hollaender knows more about this than I do. One can say 

 that there is a time in the process of cell division when radiation can no longer 

 stop it. 



HOLLAENDER: Yes, that is true. 



PATT: If the cell has begun to divide, it usually completes the division 

 process. 



