welcome to Cell division and cell cycle
Phase G1: The cyclin D/Cdk4 complex assembles at the beginning
of G1; the cyclin E/Cdk2 complex assembles near the end of G1 as the cell is
preparing to cross checkpoint 1 to start DNA synthesis (during S phase).
Phase S: The cyclin A/Cdk2 complex assembles as DNA synthesis starts.
Phase G2: Completion of G2 is indicated by the
assembled cyclin A/Cdk1 complex.
Phase Mitosis: A cell crosses checkpoint 2 to initiate mitosis when the
cyclin B/Cdk1 complex assembles. The cyclin B/Cdk1 complex is degraded by the 26S proteasome and an
assembled cyclin D/Cdk4 marks the start of the G1 phase of a new cell cycle.