The cell cycle
What is the cell cycle?
The cell cycle is comprised of four ordered, strictly regulated phases referred to as G1 (gap 1), S (DNA synthesis), G2 (gap 2) and M (mitosis/meiosis). Normal cells grown in culture will stop proliferating and enter a quiescent state called G0 once they become
confluent or are deprived of serum or growth factors. The first gap phase (G1) prior to the initiation of DNA synthesis, represents the period of commitment that separates M and S phases as cells prepare for DNA duplication. Cells in G0 and G1 are receptive to growth signals but once they have passed a restriction point they are committed
to enter DNA synthesis (S phase). Cells demonstrate arrest at different points in G1 in response to different inhibitory growth
signals promote progression through G1 to S phase utilising phosphorylation of the retinoblastoma gene product (pRb). Following DNA synthesis there is a second gap phase (G2)
prior to mitosis (M) allowing cells to repair errors that have occurred during DNA replication and thus preventing propagation of these errors to daughter cells. Although the duration of individual phases may vary, depending on cell and tissue type, most adult cells are in a G0 state at any one time.