In one experiment, the two scientists fused a mammalian cell in G1 (before the restriction point) with a mammalian cell in S phase. In the fused cell the two nuclei are visible, and it was observed that the fusion caused the G1 nucleus to enter S phase. From your knowledge about the cell cycle control of the initiation of DNA replication, what components present in the G1 nucleus and the S phase cell were responsible for the G1 nucleus initiating DNA replication in the fused cell?