RNA synthesis in relation to DNA replication in synchronized Chinese hamster cell cultures

Selection and reversal of Colcemid‐blocked mitotic cells yielded synchronous populations of 90% purity, representing 10% of the original culture. This system gave sufficient material to allow the isolation of RNA, pulse‐labelled with tritiated uridine, in various stages of the cell cycle. Sucrose gradient sedimentation of such isolations showed that incorporation of the isotope into the most rapidly sedimentating RNA fractions was greater in S and G2 than it was in G1. In addition, the total amount of labelled RNA in a given pulse was greater following the initiation of DNA synthesis. The rate of RNA synthesis increased following mitosis and reached an initial plateau after one hour. A second increase in rate was observed three hours later, or about one hour after the beginning of DNA synthesis. Several peaks in synthetic rate seemed to be superimposed on the overall increase. Short pulse times appeared to accentuate the discontinuous nature of the rate curve, while longer pulse times made the cyclic peaks more obvious. Inhibition of DNA synthesis with 1 mM thymidine prevented the change in RNA synthetic rate from occurring. The doubling in RNA synthetic rate is interpreted to be the result of replication of the functional DNA templates. The periodic fluctuations in rate suggest that, although the potential for RNA synthesis is doubled with DNA replication, the rate at any instant is not totally determined by the gene dosage.