The cell can divide only if DNA replication is free of errors or has been repaired, if the cells have grown to the right size, if the chromosomes are in the right place during mitosis.This way, two identical daughter cells are created and the daughter cells are identical to the parent cell.Checkpoints control the cell cycle by verifying if the processes at each point of the cell cycle are complete and error free. If so the cell is allowed to proceed to the next stage.G1 checkpoint at the end of the gap1 phase checks the cell is not damaged before it enters the S phase. If the cell is ok the DNA is replicated and if not it enters the G0 phase.G2 checkpoint is before the start of mitosis. The DNA needs to have been replicated without errors.The Metaphase checkpoint is also known as the spindle assembly checkpoint. The chromosomes are assembled on the metaphase plate attached to the spindles by their centromeres in the correct positions.Enzymes called kinases attach a phosphate group to proteins (phosphorylation). Adding a phosphate group activates certain proteins in the cell cycle.Proteins called cyclins help to control the checkpoints. When the correct cyclin binds to the correct kinase a CDK or cyclin dependent kinase is formed. These CDK complexes are activated by enzymes and catalyse the activation of cell cycle proteins by phosphorylation.Different enzymes break down the cyclins when they are not needed. The break down of a cyclin signals the cell to proceed to the next part of the cycle.Overexpression of a cyclin gene leads to uncontrolled cell division.