The spatial position and replication timing of chromosomal domains are both established in early G1 phase

Mammalian chromosomal domains replicate at defined, developmentally regulat ed times during S phase. The positions of these domains in Chinese hamster nuclei were established within 1 hr after nuclear envelope formation and ma intained thereafter. When Gl phase nuclei were incubated in Xenopus egg ext racts, domains were replicated in the proper temporal order with nuclei iso lated after spatial repositioning, but not with nuclei isolated prior to re positioning. Mcm2 was bound both to early- and late-replicating chromatin d omains prior to this transition whereas specification of the dihydrofolate reductase replication origin took place several hours thereafter. These res ults identify an early G1 phase point at which replication timing is determ ined and demonstrate a provocative temporal coincidence between the establi shment of nuclear position and replication timing.