During G1, the cell accumulates enzymes
   and  molecules  required  for  DNA  replication, which occurs in the S
   phase. During G2, the cell continues to grow and prepare for division.
   Differentiated cells enter the G0 phase and stop dividing.

   There are three checkpoints that must be passed for a cell to complete
   the cell cycle.

   At  the  G1/S checkpoint, the cell monitors its size and determines if
   its DNA has been damaged.

   At the G2/M checkpoint, the cell monitors DNA damage from the S phase.

   At  the  M  checkpoint,  the cell monitors spindle formation and their
   attachment to kinetochores.

   Cancer   cells   are   unable   to   enter  G0  and  become  quiescent
   appropriately.