The incorporation of 5-3H-uridine and 5-3H-cytidine into nucleolar and nonnucleolar RNA in the nucleus of monkey and pig kidney cells was measured in vitro during the cell life cycle. Time-lapse cinematographic records were made of cells during asynchronous exponential proliferation, in order to identify the temporal position of individual cells in relation to the preceding mitosis. Immediately following cinematography, cells were labeled with uridine-3H and cytidine-3H for a short period, fixed, and analyzed by radioautography. Since the data permit correlation of the rate of RNA labeling with the position of a cell within the cycle, curves could be constructed describing the rate of RNA synthesis over the average cell cycle. RNA synthesis was absent in early telophase, and rose very abruptly in rate in late telophase and in very early G1 in both the nucleus and the reconstituting nucleolus. Thereafter, through the G1 and S periods the rate of nuclear RNA synthesis rose gradually. When we used a 10-min pulse, there was no detectable change in the rate for nucleolar RNA labeling in monkey kidney cells during G1 or S. When we used a 30-min labeling time, the rate of nucleolar RNA labeling rose gradually in pig kidney cells. With increasing time after mitosis, the data became more variable, which may, in part, be related to the variation in generation times for individual cells.
Tag‐Free Internal RNA Labeling and Photocaging Based on mRNA Methyltransferases
