Very late DNA replication in the human cell cycle

G(2) was defined originally as the temporal gap between the termination of DNA replication and the beginning of mitosis. In human cells, the G(2) period was estimated to be 3–4 h. However, the absence of replicative DNA synthesis during this period designated G(2) has never been shown conclusively. In this report, we show that, at some autosomal and X linked loci, programmed DNA replication continues within 90 min of mitosis. Furthermore, the major accumulation of cyclin B1, a cell-cycle marker that is usually ascribed to G(2), overlaps extensively with very late DNA replication. We conclude that the G(2) period is much shorter than previously thought and may, in some cells, be nonexistent.