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 expected 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 last. However, the cell cycle is now depicted as falling dominoes: The preceding phase has to "fall" before the next phase can begin. 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 cell division's integrity.
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:
Read more about this topic: Cell Cycle Checkpoint
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