Very late DNA replication in the human cell cycle

Abstract

G₂ was defined originally as the temporal gap between the termination of DNA replication and the beginning of mitosis. In human cells, the G₂ period was estimated to be 3–4 h. However, the absence of replicative DNA synthesis during this period designated G₂ 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 G₂, overlaps extensively with very late DNA replication. We conclude that the G₂ period is much shorter than previously thought and may, in some cells, be nonexistent.