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driving at, at the moment. I have followed thymic weight in irradiated mice in 

 studies of the development of lymphoid tumors. In normal animals, the thymus 

 gradually decreases in weight over an age period which runs about 100 days. If 

 we irradiate these animals the thymic weight will fall to a small fraction of the 

 normal, perhaps to 10 or 20 mg. , whereas the normal then is about 50 to 60 mg. 



Let's take the case for fractionated periodic irradiation. The thymus 

 will be reduced to this weight and then will stay at this low level with some minor 

 wiggles for a matter of 50 to 70 days and will begin finally to grow, but at this 

 point it already has a tumor in it. If we irradiate while shielding this animal's 

 hind leg or inject it with bone marrow -- within 4 days after the last irradiation, 

 thymic weight shoots upward and very shortly exceeds the normal, then settles 

 back to the baseline, and no tumors form. We have followed both DNA and RNA 

 per cell in unirradiated controls and in groups irradiated with and without thigh 

 shielding at a series of intervals to 100 days. There is no change in DNA per 

 cell at any of these intervals with the exception of a small increase beyond 100 

 days when the tumors appear. This has been measured either chemically on cell 

 suspensions where we count the cells or histochemically using the Feulgen stain 

 on imprints, so that we have both population and individual cell determinations. 



For RNA, on the other hand, there is a pattern that is of interest. The 

 RNA per cell stays reasonably constant for the unirradiated thymus. Within 4 

 days after irradiation there is a 300 or 400 percent increase in RNA per cell. It 

 does not matter whether the cells are from shielded or unshielded' animals . But 

 in the shielded animals or in those receiving bone marrow, this falls promptly 

 to normal and stays there. In the unshielded animals, the thymus cannot regen- 

 erate and yet, the RNA per cell stays up at these grossly abnormal levels clear 

 as long as 100 days. Thus, the thymic cells of these animals have accumulated 

 RNA but they are unable to divide. 



SPIEGELMAN: We have some unpublished experiments with different 

 materials that agree with these results. We tried to be very cute and to force 

 the cell to make a lot of RNA, in the hope that we could then demonstrate that 

 such cells could make enzymes more effectively. Indeed, we went further and 

 tried to induce the synthesis of specific kinds of RNA. The attempt went along 

 the following lines: If microorganisms are incubated in the presence of an 

 amino acid analogue and a mixture of amino acids, they are unable to synthesize 

 protein, but can form RNA. One can thus obtain cells with as much as 4 times 

 the normal RNA content per cell. Such incubations were carried out in the pres- 

 ence of a specific inducer of the p-galactosidase system of E.coli. After the in- 

 cubation was over, the amino acids antagonist was reversed by adding the cor- 

 responding homologue. One finds that the accumulation of RNA does little for en- 

 zyme synthesizing capacity, indeed, quite the contrary. The cells grow much 

 more slowly than corresponding controls and they show little ability to form en- 

 zyme for quite a while. In fact, they continue this relatively poor physiological 

 behavior until the RNA that they have accumulated is diluted out. This result may 

 simply mean that the wrong kind of RNA has been synthesized, and this may be 

 the situation described here today. 



TOBIAS: Dr. Kaplan, presumably irradiation somehow inactivates the 

 RNA that is there and more is needed. 



JONES: Chick embryo RNA protein is a required factor for culture of 

 the chick fibroblast. 



KAPLAN: This goes back to Dr. Chargaff's idea that we should not be 

 talking about RNA as if it were something discrete. RNA is a collection of differ- 



