Apoptosis, or programmed cell death (PCD), is a selective process for the removal of unnecessary, infected or transformed cells in various biological systems. As it plays a role in the homeostasis of multicellular organisms, apoptosis is tightly regulated through two principal pathways by a number of regulatory and effector molecules.
n proliferating cells, the cell cycle consists of four phases. Gap 1 (G1) is the interval between mitosis and DNA replication that is characterized by cell growth. Replication of DNA occurs during the synthesis (S) phase, which is followed by a second gap phase (G2) during which growth and preparation for cell division occurs. Together, these three stages comprise the interphase phase of the cell cycle. Interphase is followed by the mitotic (M) phase.