DNA replication in synchronized cultured mammalian cells. VI. The temporal replication of ribosomal cistrons in synchronized cell lines

The time of synthesis of ribosomal genes was studied in a haploid (Rana pipiens), and a pseudodiploid (Chinese hamster) cell line. R. pipiens cells were synchronized by amethopterin block. Chinese hamster cells were synchronized by isoleucine starvation followed by hydroxyurea treatment. DNA replicated during three or four selected intervals of the S period was separated from the remainder of the DNA by bromodeoxyuridine density labeling. Purified bromodeoxyuridine substituted DNA was annealed with radioactive-labeled 28S ribosomal RNA (rRNA) to determine when, during different intervals of S, the nuclear DNA homologous to rRNA was replicated. In the R. pipiens and Chinese hamster cell lines, the percent of nuclear DNA homologous to 28S rRNA is highest in the DNA replicated during the first half of the S period.