Fixed time is required for all of the following steps of cell cycle; except

•  The time taken for S, G2 and M phases are similar for most cell types, occupying about 6, 4 and 2 hours respectively. •  The duration of G1 shows considerable variation. It can be as short as 2 hours in rapidly dividing cells like embryonic tissues or as long as 12 hours in some adult tissues. •  G1 phase is most variable because, in this phase cells are not committed to DNA replication. They can either enter resting state or progress to next cell division.