Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic cells. These checkpoints verify whether the processes at each phase of the cell cycle have been accurately completed before progression into the next phase. Multiple checkpoints have been identified, but some of them are less understood than others.
G1 (Restriction) Checkpoint
Main article: restriction point
The first checkpoint is located at the end of the cell cycle's G1 phase, just before entry into S phase, making the key decision of whether the cell should divide, delay division, or enter a resting stage. Many cells stop at this stage and enter a resting state called G0. Liver cells, for instance, only enter mitosis around once or twice a year. The G1 checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into G0 for an extended period. In animal cells, the G1 phase checkpoint is called the restriction point, and in yeast cells it is called the start point. The restriction point is mainly controlled by action of the CKI- p16 (CDK inhibitor p16). This protein inhibits the CDK4/6 and ensures that it can no longer interact with cyclin D1 to cause the cell cycle progression. In growth induced or oncogenic induced cyclin D expression, this checkpoint is overcome because the increased expression of cyclin D allows its interaction with CDK4/6 by competing for binding. Once active CDK4/6-CYCLIN D complexes form, they phosphorylate the tumour suppressor retinoblastoma (Rb), this relieves the inhibition of the transcription factor E2F. E2F is then able to cause expression of cyclin E, which then interacts with CDK2 to allow for G1-S phase transition. That brings us to the end of the first checkpoint which allows the G0-G1-S-phase transition.
 G2 Checkpoint
The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitosis). In order for this checkpoint to be passed, the cell has to check a number of factors to ensure the cell is ready for mitosis. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis. The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "Maturation promoting factor" (or Mitosis Promoting Factor, MPF). The MPF activates the CDK in response to environmental conditions being right for the cell and allows the cell to begin DNA replication.
The molecular nature of this checkpoint involves an activating phosphatase, known as Cdc25, which under favourable conditions removes the inhibitory phosphates present within the MPF complex. However, DNA is frequently damaged prior to mitosis, and to prevent transmission of this damage to daughter cells, the cell cycle is arrested via inactivation of the Cdc25 phosphatase (via phosphorylation with other protein kinases).
 Metaphase Checkpoint
Main article: Spindle checkpoint
The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes have/should have aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed and this is what initiates the anaphase entry. This sensing mechanism allows the degradation of cyclin B which harbours a D-box (destruction box). Degradation of cyclin B ensures it no longer inhibits the anaphase promoting complex, which in turn is now free to breakdown securin. The latter is a protein whose function is to inhibit separase, the protein composite which is responsible for the separation of sister chromatids. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separin then causes sister chromatid separation. After the cell has split into its two daughter cells, the cell enters G1.