Two phases of DNA replication in human cells.

The course of DNA synthesis in the chromosomes was studied in synchronized human lymphocyte cultures, by means of the BrdU-Hoechst-Giemsa method. In comparing replication patterns and G-banding it was found that with regard to banding the process of DNA replication can be divided into two separate phases, an "early replication period" which is characterized by DNA synthesis in R bands of the autosomes and active X chromosome, and a "late replication period" which concerns the G-positive regions of the autosomes and all the bands of the heterochromatic X and Y chromosomes. No overlapping was found between the two phases mentioned. The possible role of regulatory mechanisms was discussed.