Prevents cells to go forwards when its not ready. Mpf = maturation promoting factor = cyclin + cdk complex. G1/s cyclins (e): bind their cdk(2) near end of g1 and lead cell into dna replication. S-cyclin (a): bind cdk (20 during s phase and are required for dna replication. M- cyclins (a,b) cdk(1) to promote the event of mitosis. G1-cyclins (d): (in most cells) bind cdk (4,6) to promote passage through restriction point in late1. Quiescence cells are differentiated by cell cycle is arrested in g0 this can be permanent or temporary. Dna damaged; g0 and cell repair activated dna repaired; resume g1/s or quiescence. P53 various proteins cdk2 s phase or cdk 1 m phase. Dna is still uncondensed but duplicated, intacted nuclar envalop spindles will develop and pull away create daughter chromatids. Checkpoint is at the end of m phase end of metaphase.