80 



ZIRKLE: That depends on the kind of cells and on conditions. With 

 suitable doses, cells irradiated during division can be stopped at various stages. 



PLATZMAN: There appears to be a substantial spread in the times 

 for division. This may be a significant fact, 



MAZIA: The spread is relatively small under the same conditions. A 

 few hours between the first cell division and the last cell division. 



PLATZMAN: Is it approximately a relative curve? 



Given exactly the same experimental conditions, what differences are 

 there in division times for a given kind of cell? 



MAZIA: About 15 or 20 percent. 



TOBIAS: Taking yeast cells, the fluctuation in time for cell division is 

 about + 8 percent at 30°C. At higher temperatures, the relative uncertainty 

 in time increases, 



PLATZMAN: It seems to me that this is a significant thing to think 

 about with regard to the determining factors, 



TOBIAS: Yes, after irradiation the fluctuation of cell division times 

 increases as the time for cell division is prolonged. 



Careful data have been taken by Victor Burns at Berkeley on the rela- 

 tionship of cell division delay to the fluctuation in time for cell division. Both 

 are more or less proportionately increased by a small dose of radiation. We in- 

 fer that the chemical order of reaction did not change much but that the time con- 

 stants became slower. It is also interesting to note that cell division delay is 

 longer for the second division following irradiation than for the first division. I 

 do not know whether or not the fluctuation in cell division time is directly relat- 

 ed to the steep rise of RNA content just before division. 



We might finish our model for sublethal radiation damage. I would do 

 that by having the RNA react back on the system initiating its production, that is, 

 on the DNA; thus cell division would be triggered. 



KAPLAN: You are suggesting that with the mere accumulation of RNA 

 in the cell mass it divides, 



MAZIA: It may not be that simple. RNA is certainly heterogeneous, 

 and we might require a certain amount of a certain kind of RNA to realize the 

 trigger reaction. 



SHERMAN: In this model, is there an interference with DNA produc- 

 tion in irradiated cells? 



MAZIA: Yes. 



SHERMAN: So that the DNA that is already present continues to pro- 

 duce RNA. This accumulates because a link in the feedback loop has been bro- 

 ken. 



KAPLAN: Well, RNA might pile up in that way, but certainly the 

 mere accumulation of RNA does not make the cell divide. That is what I am 



