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.
Cells can exit G0 when
signals such as growth factors and hormones bind to surface receptors;
signal transduction pathways then carry the signals to the nucleus,
leading to abnormal gene expression.