oint in which cells cannot move to S phase for replication if they don't meet the requirements, what other things need to occur for S phase to happen?...

During the cell cycle G1 has a restriction point in which cells cannot move to S phase for replication if they don't meet the requirements, what other things need to occur for S phase to happen? Including a need for active maturation promotion factor (MPF). Why is MPF only active S phase, G2, and into Metaphase and becomes inactive in Anaphase?