It makes sense that G1 cells would be able to replicate while G2 cell would not be able to. Even when a signal for replication is present, G2 will not replicate. I would think that the G2 cells must have a signal or something that indicates that it has passed certain checkpoints, such as the DNA replication checkpoint at the end of S phase. The ORC is a multi-subunit SNA binding complex that binds to the orgins of replication in all eukaryotes. It is made up of 6 subunits and ATP is needed for binding to occur. The subunits are encoded by ORC 1, 2, 3, 4, 5, and 6. The ORC is active at the end of mitosis and early G1, which makes sense for replication. While it is highly unlikely since so many different complexes and proteins are involved, what happens if re-replication occurs after S phase? Is this even possible at all? It makes sense that Cdk activity reduces to zero so chromosomes are ready for a new round of replication. It reminds me of a "reset" button.
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