 Association of cyclin A with CDK2 is required for passage into S phase while association with CDK1 is required for entry into M phase.Cyclin A resides in the nucleus during S phase where it is involved in the initiation and completion of DNA replication.As the cell passes from G1 into S phase, cyclin A associates with CDK2, replacing cyclin E. Cyclin E is responsible for initiating the assembly of the pre-replication complex.