cubation with tritiated cytidine were found to be utilized at a later time for 

 DNA synthesis. 



4. The majority of the incorporated cytidine is recovered in the RNA. 

 The label in the acid soluble pool déclines steadily with time after cessa- 

 tion of incorporation of the labeled cytidine, while H' RNA was stabil. 



5. It is confirmed for HeLa cells that cytidine is in part converted to 

 thymine for DNA synthesis and uridine for RNA synthesis. 



The expérimental results lend support to the hypothesis that RNA syn- 

 thesis begins in the chromatin portion of the nucleus. The fate of the 

 incorporated cytidine apparently is decided in the nucleus, where it is 

 first linked specifically and from where it progresses to the building 

 blocks of nucleolar and cytoplasmic RNA or DNA. 



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