Mitogens and the Cell Cycle

Unicellular organisms, such as bacteria, divide when there are enough nutrients in the environment.

However, in multicellular organisms, most cells remain in G0, or the non-dividing phase, until cell division is triggered by extracellular signal molecules, called 'mitogens’.

Mitogens are usually small proteins or peptides secreted in response to different stimuli, such as tissue injury, infection, or a routine regeneration.

For instance, in the case of tissue injury, specialized cells secrete a mitogen called Platelet-derived growth factor or PDGF.

Such mitogens can bind to the extracellular domain of tyrosine kinase receptors such as PDGF-Receptor, inducing receptor dimerization and autophosphorylation of its intracellular domains. This allows phosphorylation and activation of the downstream intracellular molecules that are involved in multiple signaling pathways.

The mitogen-activated protein kinase or MAP kinase cascades are a type of signaling pathway induced by mitogen-receptor binding. This pathway starts with the activation of the small membrane bound GTPase named Ras by the receptor.

Next, active Ras triggers the MAP kinase cascade which includes a series of kinase proteins