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RNA by the nucleus under the conditions of the experiment. If we assume that 

 the cytoplasmic RNA is breaking down at the same rate in the whole ameba as in 

 the half ameba, we can calculate the rate of loss of cytoplasmic RNA in the 

 whole ameba. It will lose twice as much, in a given time, as the half ameba, 

 because there is twice as much to start with. But if the nucleus can synthesize 

 it only at the rate calculated from the data on the nucleated half-ameba, obvious- 

 ly it will not be able to keep up with the loss that is occurring in the whole ame- 

 ba; hence, it will suffer a net loss, as observed. Using the actual quantitative 

 values, we should be able to predict the curve for RNA loss from the whole ame- 

 ba from the rates of loss and replacement calculated from the data on the halves. 

 The predicted curve corresponds very well to the experimental curve, and there- 

 fore, I feel that these experiments strengthen the theory -- which has much oth- 

 er support -- that the RNA of the cytoplasm, which is most of the RNA of the 

 cell, originates in the nucleus. 



SPIEGELMAN: Have you followed the turnover of RNA in these enu- 

 cleated ameba? 



MAZIA: Yes. At least radioactive phosphorus data indicate that it is 

 only 1/3 that of the nucleated part. 



SPIEGELMAN: But Brachet claimed that there was considerable ca- 

 pacity for reformation of RNA even in the enucleated cells. 



MAZIA: The figure is l/3. The experiments of James ^how, however, 

 that there is a net loss in the absence of the nucleus. 



MAGEE: I don't understand the relation between the RNA in this cell 

 and the one that you were talking about previously. 



MAZIA: Earlier, I was speaking about a growing cell, one that was 

 taking in food. The data of Brachet and of James are for starving cells. We 

 used the starving cells in order to eliminate variation due to food intake, but, in 

 fact, it is the simplification introduced by starvation that makes the evidence for 

 the nuclear origin of cytoplasmic RNA so clean. 



KAMEN: There are no DNA data on these cells? 



MAZIA: Unfortunately, no. 



TOBIAS: If you will apply these ideas to my question raised earlier, 

 then one possible explanation for a delay in cell division after a small dose of 

 radiation seems to be that the irradiation would inactivate a good deal of the RNA 

 in the cell; it would take quite a while for the DNA to resynthesize enough RNA 

 for division to get going. You may recall that there is some evidence for in- 

 creased RNA in irradiated tissues. 



MAZIA: There are some radiation data on ameba that Hirshfield and I 

 published a few years ago that may be relevant, although we were dealing with 

 UV irradiation. We measured the delay of division caused by UV and compared 

 the effect of a given dose in delaying the division of a whole cell with the effect 

 on a cell from which half the cytoplasm had been removed. We found that when 

 the cytoplasmic volume was reduced by one half, the radiosensitivity (measured 

 as delay of division) doubled; it took half the dose to produce the same effect. 

 This would fit your proposal perfectly. 



SPIEGELMAN: Isn't it true that you hit DNA synthesis first? 



