Cell cycle checkpoint

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, though some of them are less understood than others.

Function
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 either to stall the cell cycle until repairs are made or, if repairs cannot 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 were the case, it would be expected that the stages of the cell cycle must function according to some sort of internal clock, which would determine how long a phase should take. However, the cell cycle is now depicted like the falling dominoes: The preceding phase has 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 that all play salient roles in the maintenance of the integrity of the division.

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. Cells that continually divide in the adult human include hematopoietic stem cells and gut epithelial cells. Therefore, the re-entrant into the cell cycle is possible only 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 that 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. Many cells stop at this stage and enter a resting state called G0. Liver cells, for instance, enter mitosis only 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 controlled mainly 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), which 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.

In simpler terms, the CDK inhibitor p16 inhibits another CDK from binding to its cyclin (D). When growth is induced, the expression of this cyclin is so high that they do bind. The new CDK/cyclin complex now phosphorylates retinoblastoma (a tumour suppressor). Un-phosphorylated retinoblastoma inhibits a transcription factor. This factor then brings about the 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" (Mitosis Promoting Factor, MPF).

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. This is done by the ATM kinase protein which phosphorylates Cdc25 which leads to it's ubiquitinylation and destruction.

Metaphase Checkpoint
The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes have/should aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed, which initiates the anaphase entry. To do this, the sensing mechanism ensures that the anaphase-promoting complex (APC/C) is no longer inhibited, which is now free to degrade cyclin B, which harbours a D-box (destruction box), and to break down securin. The latter is a protein whose function is to inhibit separase, which in turns cuts the cohesins, the protein composite responsible for cohesion of sister chromatids. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separase then causes sister chromatid separation. After the cell has split into its two daughter cells, the cell enters G1.