Editing G1 phase (section)

===The G<sub>1</sub>/S checkpoint===

{{Main|Cell cycle checkpoint}}

The '''G<sub>1</sub>/S checkpoint''' is the point between G<sub>1</sub> phase and the S phase in which the cell is cleared for progression into the S phase. Reasons the cell would not move into the S phase include insufficient cell growth, damaged DNA, or other preparations have not been completed.

At the G<sub>1</sub>/S checkpoint, formation of the G<sub>1</sub>/S cyclin with Cdk to form a complex commits the cell to a new division cycle.<ref name=Morgan /> These complexes then activate S-Cdk complexes that move forward with [[DNA replication]] in the S phase. Concurrently, [[anaphase-promoting complex]] (APC) activity decreases significantly, allowing S and M cyclins to become activated.

If a cell does not clear to pass through to the S phase, it enters the dormant G<sub>0</sub> phase in which there is no cellular growth or division.<ref name=Lodish />