Apoptosis is defined as "programmed cell death". There are five essential steps involving apoptosis:
- Shrinkage of overall cell size accompanied by chromatin condensation
- Cell membrane blebbing
- Collapsed cell nucleus
- Formation of apoptotic bodies
- Lysis of apoptotic bodies
Apoptosis is an integral aspect of cellular processes during embryonic development.
As described earlier, if damaged DNA is encountered during the G1 checkpoint of the cell cycle, the cell will initiate DNA repair to fix the problem. If the damage is beyond repair, the cell would activate the apoptotic pathway.
p53 is a checkpoint protein and a tumor suppressor that identifies DNA damage and initiates DNA repair processes. Once DNA damage is assessed as "irreversible," p53 initiates the apoptotic pathway by the formation of apoptome which is a complex protein machinery that leads to programmed death of unwanted cells.
A notable mechanism by which cancer cells evade apoptosis is to disrupt p53 function. By inactivating p53, the guardian of cell division integrity and the initiator of apoptosis, cancer cells ensure uncontrolled cell growth. Not surprisingly, p53 mutations and deletions are observed in a number of cancer types.