The incorporation of methionine-methyl-14C into 18S ribosomal RNA of cultured Chinese hamster ovary cells in early and late interphase has been determined by zone-sedimentation analysis of phenol-extracted RNA preparations. Synchronized cell cultures were prepared for these studies by thymidine treatment and by mechanical selection of mitotic cells. The specific activity of 18S RNA labeled in late interphase was found to be 1.1–1.2 times that of 18S RNA labeled in early interphase. Upon correction for increase in RNA mass, the rate of methylation of 18S RNA in late interphase is about 1.9 times that in early interphase.

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