RNA AND DNA METABOLISM 

 IN HUMAN TISSUE CULTURE 



CELLS STUDIED 

 WITH TRITIATED CYTIDINE 



L. FEINENDEGEN, V. P. BOND, W. W. SHREEVE 

 and R. B. PAINTER 



Médical Research Center, Broc khaven National Laboratory, 

 Upton, L. I., New York 



The autoradiographic technique combined with biochemical methods was 

 employed to study aspects of RNA and DNA metabolism in human tissue 

 culture cells, incubated with tritiated cytidine. 



1. Tritiated cytidine was incorporated within minutes into ail intact 

 nuclei of HeLa S' cells and Osgood leukemic cells. The label subse- 

 quently increased over the nucleoli and over the cytoplasm. 



2. This séquence of labeling was defined and quantitatively observed 

 in turnover studies, in \^ich| the cultures were incubated with the labeled 

 nucleoside for a short time only followed by growth in carrier cytidine 

 supplément ed médium. While nuclear label decreased, cytoplasmic label 

 became visible and most nucleoli temporarily accumulated label with 

 a maximum at 1 hour after cessation of incubation with tritiated cytidine. 



Desoxy cytidine as carrier in the new médium failed to prevent further 

 uptake of tritiated cytidine, which was apparently not removed by washing 

 of the cultures. It is concluded, therefore, that the cytidine incorporation 

 dépends on enzyme Systems, différent for that of desoxycytidine. Prepara- 

 tory steps for autoradiography of the cells extracted an appréciable amount 

 of activity, which. declined after cessation of the incubation with tritiated 

 cytidine. Some label was obviously also extracted from the chromatin 

 portion of the nucleus. 



RNA synthesis stops while the cell is in nitosis as evidenced by the 

 lack of incorporation of tritiated cytidine into dividing cells. 



Evidence is presented indicating that the tissue culture cells syn- 

 thesize RNA, also during DNA synthesis period. 



3. Tritiated cytidine is incorporated into DNA during DNA synthesis. 

 Compounds not jreviously incorporated into DNA during short term in- 



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