The cellular activation of the caspase cascade resulting in cell death is triggered by chemical damage to DNA which stimulates a sequence resulting in the cleavage of Bid in a manner similar to the binding of so called death-receptors or directly initiates the permeability transition of the mitochondrial membrane. The permiability transition releases several factors including cytochrome c, AIF and other factors in to the cytoplasm. Cytochrome c, a key protein in electron transport, is released from mitochondria in response to apoptotic signals, and activates Apaf-1, a protease released from mitochondria. Activated Apaf-1 activates caspase-9 and the rest of the caspase cascade. The caspases are a class of cysteine proteases that includes several representatives involved in apoptosis. The caspases convey the apoptotic signal in a proteolytic cascade, with caspases cleaving and activating other caspases that then degrade other cellular targets that lead to cell death.
