Comparison of 3H-cytidine and 3H-5-uridine as precursors of RNA☆

Abstract The relative incorporation into RNA and DNA of 3 H-5-uridine, 3 H-cytidine and 3 H-6-thymidine was determined in brain, liver, kidney and spleen of the mouse as well as in neoplastic cell cultures. In systems with a high proliferative activity (spleen, cell cultures), the specificity of 3 H-5-uridine as a precursor of RNA was higher than that of 3 H-cytidine, although not as high as the specificity of 3 H-thymidine as a precursor of DNA. In organs with a low proliferative activity (brain, liver and kidney), the results based on a biochemical analysis did not permit any conclusions as to the distribution of label from 3 H-5-uridine and 3 H-cytidine between RNA and DNA of the small proportion of DNA-synthesizing cells.