Fig. 7.

Establishment of a program for replication of mammalian nuclei. When CHOC 400 cells, blocked in metaphase with nocodazole, are released into G1-phase, nuclear assembly is completed within 1 hour [(Dimitrova and Gilbert, 1999; Dimitrova et al., 1999) and this study], coincident with the reactivation of general transcription (D.S.D., unpublished) and the licensing of chromatin for replication (this study). Within the next hour, chromosomal domains are directed to distinct and reproducible nuclear locations and by 2 hours into G1-phase (TDP, timing decision point) the temporal order for their replication in the upcoming S-phase is established (Dimitrova and Gilbert, 1999). At the ODP (5–6 hours in G1), specific chromosomal sites, where replication will initiate, are selected within the DHFR locus [(Wu and Gilbert, 1996) and this study]. Several hours later, at the G1/S transition (12–14 hours post-metaphase) the rise in S-phase promoting factor activity (S-phase cyclin-dependent kinases and dbf4/cdc7 kinase) leads to the assembly of replication factories and the firing of the earliest replication origins (Dimitrova et al., 1999). The four major events, which prepare the genome for replication, are highlighted in red.