The checkpoint are used at each stage of the cell cycle are working correctly and if errors are found they are immediately repaired. 

(Khan Academy, Cell Cycle Checkpoints, 2008)

• This checkpoint happens at the end of G1 phase. 

• This checkpoint makes sure that each cell has enough resources and that DNA is correct before it moves onto next stage.

• It the conditions are not favorable, the cell enters G0 phase also known as resting phase.

(Khan Academy, Cell Cycle Checkpoints, 2008)

• This checkpoint happens during S phase

• This checkpoint makes sure that all the DNA is replicated complete and accurate. This checkpoint is similar to G2 Checkpoint.

• If there is an error, the cell stops division and tries to either repair the damage or go through apoptosis. 

(Khan Academy, Cell Cycle Checkpoints, 2008)

• This checkpoint happens at the end of G2 phase.

• This checkpoint verifies the DNA that was replicated is completely and correctly replicated.

• If there is a error, the cell halts division for repair or undergoes apoptosis.

(Khan Academy, Cell Cycle Checkpoints, 2008)

• This checkpoint occurs during metaphase.

• This makes sure all chromosomes are properly attached to spindle fibers before going to the next phase (anaphase).

• Having this checkpoint will help prevent anaphase from occuring. If anaphase were to occur when the spindle fibers correctly attched it will lead to a unequal amount of chromosome distribution.

(Khan Academy, Cell Cycle Checkpoints, 2008)