An example of an external signal that regulates cell division is a growth factor. Growth factors are proteins or hormones that bind to specific receptors on the cell surface and stimulate cell proliferation.
1. Binding to Receptors : The growth factor, circulating in the extracellular fluid, binds to its specific receptor on the surface of a target cell. These receptors are typically transmembrane proteins.
2. Signal Transduction : The binding triggers a signal transduction pathway within the cell. This involves a series of molecular changes that relay and amplify the signal from the receptor to the interior of the cell.
3. Activation of Cellular Responses : The signal eventually reaches the nucleus and other parts of the cell, leading to the activation (or in some cases, repression) of specific genes. In the context of cell division, genes that promote the cell cycle are activated.
4. Progression through the Cell Cycle : The activation of these genes leads to the synthesis of proteins necessary for cell division, allowing the cell to pass through the various checkpoints in the cell cycle (G1, S, G2, M phases).
5. Cell Division : If all conditions are favorable and the necessary proteins are present, the cell will proceed to mitosis, dividing into two new cells.
A classic example of a growth factor is Epidermal Growth Factor (EGF), which binds to the EGF receptor (EGFR) on cells and stimulates them to divide. This mechanism is crucial in processes such as wound healing and tissue regeneration but can also play a role in cancer development if the regulation of cell division becomes disrupted.