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Abstract
We describe a cell-free system from HeLa cells that initiates DNA replication under
cell cycle control. G1 but not G2 phase nuclei initiate replication when coincubated
with S phase nuclei in cytosolic extracts from S phase but not from G1 or G2 phase
HeLa cells. S phase nuclei or an S phase nuclear extract are required for the initiation
of semiconservative DNA replication in G1 nuclei but not for elongation in S phase
nuclei. S phase nuclear extract could be replaced by recombinant human cyclins A and
E complexed to Cdk2 but not by Cdk2 alone or by human cyclin B1 complexed to Cdc2.
In S phase cytosol, cyclins A/Cdk2 and E/Cdk2 triggered initiation synergistically.