Many mammal cells, such as this neuron, remain permanently or semipermanently in G₀.

The G₀ phase (G sub 0) or G zero is a period in the cell cycle where cells exist in a quiescent state. G₀ phase is viewed as either an extended G₁ phase where the cell is neither dividing nor preparing to divide and or as a distinct quiescent stage which occurs outside of the cell cycle.^[1] G₀ is sometimes referred to as a "post-mitotic" state since cells in G₀ are in a non-dividing phase outside of the cell cycle; some types of cells (such as nerve cells and heart muscle cells) when they reach maturity (i.e., when they are terminally differentiated) become post-mitotic (enter G₀ phase) but continue to perform their main functions for the rest of the organism's life. Multinucleated muscle cells which do not undergo cytokinesis, are also often referred to as in the G₀ stage.^[1]

[edit] In relation to the cell cycle

Cells enter the G₀ phase from a cell cycle checkpoint in the G₁ phase, such as the restriction point (animal cells) or the start point (yeast). This usually occurs in response to a lack of growth factors or nutrients. During the G₀ phase, the cell cycle machinery is dismantled and cyclins and cyclin-dependent kinases disappear. Cells then remain in the G₀ phase until there is a reason for them to divide. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G₀ phase semi-permanently and can only be induced to begin dividing again under very specific circumstances. Other types of cells, such as epithelial cells, continue to divide throughout an organism's life and rarely enter G₀.

[edit] Distinction between senescent cells

Although many cells in the G₀ phase may die along with the organism, this does not mean that cells that enter the G₀ phase are destined to die; this is often simply a consequence of the cell lacking any stimulation to re-enter in the cell cycle.

The term "post-mitotic" is sometimes used to refer not only to quiescent cells (like those in G₀) but also to senescent cells. Cellular senescence is distinct because it is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable. Senescence then, unlike quiescent cells, is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.

[edit] References

[edit] See also

• Cell Cycle

v • d • e Cell cycle Interphase G1 phase · S phase · G2 phase M phase Mitosis (Preprophase · Prophase · Prometaphase · Metaphase · Anaphase · Telophase) · Cytokinesis Cell cycle checkpoints Restriction point · Spindle checkpoint · Postreplication checkpoint Other cellular phases G0 phase · Apoptosis