While overcoming the G1/S checkpoint to commence DNA replication requires cyclin E, and traversing the G2/M checkpoint to initiate mitosis requires cyclin B to be present, cyclin A seems to be required for both S-phase and M-phase. A number of studies have described the ability of over-expressed cyclin A to accellerate the G1 to S transition causing DNA replication, and cyclin A antisense DNA can prevent DNA replication. Cyclin A availability is apparently the rate-limiting step for entry into mitosis, and cyclin A is required for completion of prophase. At late prophase, cyclin A may no longer be necessary as cdc2/cyclinB1 becomes active.
