The Cell Cycle

Interphase
Metaphase
Mitosis

The cell cycle of a typical eukaryotic cell has four phases. The relatively brief M phase consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis). After M phase, the daughter cells each begin interphase of a new cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division. The first phase within interphase is called G1 (G indicating gap); during this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated. The cell then enters the G2 phase, which lasts until the cell enters the next round of mitosis. Metabolic activity, cell growth, and cell differentiation all occur during interphase.

The term “post-mitotic” is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell’s progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.