What does the release of cytochrome c from mitochondria signify?

The release of cytochrome c from mitochondria is a crucial event that signals the initiation of programmed cell death, or apoptosis. In a healthy cell, cytochrome c is located in the intermembrane space of the mitochondria and plays a key role in the electron transport chain, which is essential for ATP production. However, when a cell is under stress due to DNA damage, oxidative stress, or other damaging signals, the mitochondrial membrane becomes permeable, allowing cytochrome c to leak into the cytosol.

Once in the cytosol, cytochrome c binds to Apaf-1 and triggers the formation of the apoptosome, which recruits and activates caspase-9. This activation cascade ultimately leads to the execution phase of apoptosis, where specific cellular components are systematically dismantled and removed in a regulated manner. The presence of cytochrome c in the cytoplasm thus serves as a critical indicator that the cell is undergoing apoptosis rather than normal cellular processes such as repair or immune activation.

Other options, such as activation of the immune response, initiation of the cell cycle, or cellular repair, do not involve cytochrome c release, making them unrelated to its role in signaling apoptosis. Instead, cytochrome c