Apoptosis is a highly regulated process in multicellular organisms. There are two initiation pathways induce either by an internal trigger, intrinsic pathway, like cell stress or an external trigger, extrinsic pathways, e.g. specific signaling from surrounding cells. The underlying mechanism induced by both pathways is the activation of initiator caspases and the downstream activation of executioner caspases which are specific enzymes/proteases degrading proteins indiscriminately.
The program follows very characteristic changes including blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation and RNA decay. The formation of apoptotic bodies prevent the release of cellular content onto surrounding cells and are finally removed by phagocytic cells.
Apoptosis represents a very important process druing an organism's life cycle e.g. during development and homeostatsis. Defects in the apoptotic pathway have been linked to a wide range of diseases. The regulation of the apoptotic pathways by proteins promting or inhibiting apoptosis thus is a critical equilibrium to prevent atrophy caused by extensive apoptosis or uncontrolled cell proliferation resulting in diseases such as cancer.
The protein p53 has sigificant impact on that critical equilibrium. This tumor-suppressor protein usually accumulates when DNA is damaged due to a chain of biochemical factors. While repairing p53 stops the cell from replicating by stopping the cell cycle at G1 or interphase. This Process gives the cells time to repair damage, however it will induce apoptosis if damage is extensive and repair efforts fail.