The cell cycle involves the duplication of the intracellular content, followed by the division into two daughter cells.
Replication of cellular content, especially the DNA, is highly critical, and any mistake during the process can lead to conditions such as cancer.
So how do cells prevent any errors during division?
The cell cycle control system features regulator proteins that halt the cycle at various checkpoints.
At every checkpoint, the regulator proteins prevent the initiation of each step until the earlier stages are completed and any errors have been corrected.
Generally, the control system has three crucial checkpoints found in the G1, G2, and M phases.
At the G1 checkpoint, the regulator protein checks if the cell has reached the critical size and the DNA is free from errors. It also checks whether enough nutrients and growth factors are present to begin DNA synthesis.
At this point, if the cell does not receive the necessary signal, it switches to a resting state called the G0 phase until all the conditions are met.
The cells that pass the G1 checkpoint transit through the synthesis, or S-phase, when the DNA gets replicated.
After this, the cell encounters a second checkpoint at the G2 phase, where the regulator proteins check any errors in the DNA and whether the cell has an appropriate size to enter Mitosis or M-phase.
During Mitosis, the control system verifies if the chromosomes are attached to the spindle and are accurately aligned so that the cycle proceeds for cell division. If at any point the regulator protein detects irreparable damage, cell death occurs. One type of critical regulator proteins are the cyclin-dependent protein kinases, or CDK. CDKs form complexes with cyclins, and their activity affects proteins directly involved in cell growth and DNA synthesis.