G0 Phase

Some cells leave G1 and enter a non-dividing, quiescent stage called G0, where they remain metabolically active but do not proliferate; a cell can stay in G0 for a very long time, and while there it typically shows a reduced rate of RNA and protein synthesis compared with a cycling cell. The decision to enter G0 is made at the restriction point checkpoint near the end of G1, which determines whether a cell will continue through the cycle and divide, or instead withdraw into G0 as a quiescent, possibly terminally specified cell, or undergo cell death. Cells are commonly pushed into G0 by nutrient deprivation, by a lack of growth factors, or by density-dependent inhibition (crowding by neighbouring cells), all of which are read out by internal proteins - kinases and cyclins - that normally activate the genes needed for division; cyclins in particular act as the master checkpoint controller of whether a G1 cell divides at all. G0 is not necessarily a dead end: many cells re-enter the cycle when the right growth factors or extracellular signals appear later, so G0 cells are described as quiescent, not dormant. A striking exception is that mature neurons and skeletal muscle cells remain permanently in G0 and never divide again - yet even the brain retains a reserve of neural stem cells that themselves sit in G0 until they are called upon to replace neurons and neuroglia lost to injury or death, which is how limited neuron replacement is possible in an adult brain. The same G0 property was exploited to clone Dolly the sheep: resea …