Cell cycle checkpoint
Cell cycle checkpoints are "Regulatory signaling systems that control the progression through the cell cycle. They ensure that the cell has completed, in the correct order and without mistakes, all the processes required to replicate the genome and cytoplasm, and divide them equally between two daughter cells. If cells sense they have not completed these processes or that the environment does not have the nutrients and growth hormones in place to proceed, then the cells are restrained (or "arrested") until the processes are completed and growth conditions are suitable". These 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 well understood than others.
An important function of many checkpoints is to assess DNA damage, which is detected by sensor mechanisms. When damage is found, the checkpoint uses a signal mechanism to either stall the cell cycle until repairs are made or, if repairs can't be made, to target the cell for destruction via apoptosis (effector mechanism). All the checkpoints that assess DNA damage appear to utilize the same sensor-signal-effector mechanism.
The cell cycle according to Temple and Raff, 1986, was meant to function as a clock, but if this was the case it would be expected that the stages of the cell cycle must function according to some sort of internal clock, that will determine how long a phase should take. Contradictorily, the cell cycle is now depicted like the falling dominoes, the preceding phase has got to "fall" before the next phase can take place. The cell cycle checkpoints are therefore made up of composites of protein kinases and adaptor proteins which all play salient roles in the maintenance of the integrity of the division.
Checkpoints are now accepted to exist at every single point in the cell cycle. The DNA damage checkpoint is always active. Nonetheless, most human cells for example are terminally differentiated and must exit the cell cycle. There is a phase late in G1 phase called the restriction point (RP, or the restriction checkpoint); cells that should cease division exit the cell cycle and enter G0. The only cells that continually divide in the adult human include hematopoietic stem cells and gut endothelial cells. The re-entrant into the cell cycle is therefore only possible by overcoming the RP. This is achieved by growth factor induced expression of cyclin D proteins. These then overcome the G0 barrier and are able to enter the cell cycle.
The main checkpoints which control the cell division cycle in eukaryotes include:

# G1 (Restriction) Checkpoint
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. Most 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. 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.

# S Phase Checkpoint
The S Phase Cell Cycle Checkpoint is the "cell regulatory signaling system that controls progression through S phase and stabilizes the replication forks during conditions that could affect the fidelity of DNA replication, such as DNA damage or depletion of nucleotide pools".
The S Phase of the cell cycle is "following G1 and preceding G2 when the entire DNA content of the nucleus is replicated. It is achieved by bidirectional replication at multiple sites along each chromosome".

# G2 Checkpoint
The second checkpoint is located at the end of G2 phase, that can restrain the 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 CDK's associated with this checkpoint are activated by the removal of an inactivating phosphate 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).

# Mitotic 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 it 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.