during G1, cyclin increases, the G1 cyclin binds to Cdk to form an activated G1 cyclin/cdk complex. once activated, cdk functions as a protein kinase that phosphorylates other proteins that are needed to advance the cell to the next phase in the cell cycle.
For exp: certain proteins involved with DNA synthesis are phosphorylated and activated thereby allowing the cell to carry out even in S phase. when cell passes into S phase, G1 cyclin is degraded. A different cyclin, called mitotoic cyclin, accumulates late in G2, it binds to a CDK to form an activated mitotic cyclin/cdk complex, this complex phosphorylates proteins that are needed to advance into Mphase.