Mechanical analogy for the cell cycle A checkpoint in the cell cycle is a control point where stop and go–ahead signals regulates the cell cycle. The above picture depicts the G1, G2, and M checkpoints of cell cycle. The cell cycle control system has been compared to the control device of an automatic washing machine.
Like the washer's timing device, the cell cycle control system proceeds on its own, driven by a built–in clock. However, just as a washer's cycle is subject to both internal control (such as sensor that detects when the tub is filled with water) and external adjustment (such as activation of the start mechanism), the cell cycle is regulated at certain checkpoints by both internal and external controls.
A checkpoint in the cell cycle is a critical control point where stop and go–ahead signals can regulate the cycle (The signals are transmitted within the cell by the kinds of signal transduction pathways). Cyclin and the cyclin dependent kinases (CDKs) are the principle regulators of cell cycle progression through activation of cell cycle checkpoints. These are the core of the complex that composes the mechanism which drives the irreversible transitions that must be successfully negotiated for cells to successfully navigate the cell cycle and proliferate.
The cell cycle control do not apply to cancer cells. This means that cancer cells do not respond normal to the body's control mechanisms:They don't stop at the normal checkpoints .If/when they stop dividing, they do so at abnormal points. Internally, cancer cells may not need growth factors to grow and divide: They may make their own growth factor. They may have an abnormal cell cycle control system. Thus, loss of cell cycle control leads to cancerous cells.