DNA replication in synchronized cultured mammalian cells. II. Replication of ribosomal cistrons in thymidine-synchronized HeLa cells

HeLa cells synchronized by double thymidine treatment were used to determine when DNA homologous to 28 s ribosomal RNA is replicated during S (the synthetic phase of the cell cycle). DNA replicated during selected intervals of the S period was isolated from the remainder of the nuclear DNA by pulse-labeling with bromodeoxyuridine and cesium chloride density-gradient centrifugation. Hybridization of this fractionated DNA with HeLa 28 s RNA indicated that in these cells, DNA homologous to 28 s RNA replicates throughout the S period.