Homogeneously staining regions (HSRs) of a rat hepatoma cell line are not early replicating

The rat hepatoma cell line H4-IIE-C3 (H4) has homogeneously staining regions (HSRs) which contain multiple, tandemly repeated copies of ribosomal RNA (rRNA) genes. We determined the time of replication of the DNA within these HSRs autoradiographically after incorporation of [3H]thymidine and by Hoechst 33258 and Giemsa staining after 5-bromodeoxyuridine (5-BrdU) incorporation. The DNA within the H4 HSRs is not early replicating, unlike that in other HSRs. It begins replicating later than much of the other nuclear DNA, continues replicating throughout most of the S phase, and is completed 1-2 h before mitosis.