What is apoptosis?

Apoptosis is programmed cell death. It is the main pathway for physiological cell death (though not the only one, see necrosis) and involves a defined set of changes to the cell: shrinkage of the nucleus, condensing of chromatin and cleavage of DNA between nucleosomes, and shrinkage and fragmentation of the cell to form apoptotic bodies. These are then phagocytosed (engulfed and "eaten") by other cells. An apoptotic cell undergoes changes to its cell membrane, with phosphotidylserine becoming exposed on the outer layer of the membrane, which may act as an "eat me" signal for other cells.
Apoptosis is governed by cell signalling pathways involving a balance between apoptosis inhibitors such as Bcl-2 and apoptosis promoters such as Bax. When the apoptosis pathway is activated, it is executed by caspases. These elements of the pathway are conserved from nematodes to man, showing the evolutionary importance of apoptosis. Programmed cell death is important physiologically in development (webs between the fingers of embryos are removed by apoptosis), and removal of damaged or dangerous cells. T cells that react to self antigens die by apoptosis, and T killer cells induce other cells (eg those infected by viruses) to undergo apoptosis. The tumour suppressor p53 induces apoptosis if a cell has irreparable DNA damage, and mutating to evade apoptosis is a key "hallmark of cancer".