Restriction Point (R)

  • Growth factor/mitogenic signaling is required for G1 phase progress. Withdrawal of growth factors/mitogens from cell culture can arrest the cells in G1 phase called G0.
  • However, at some point in the cell cycle, cells do not need growth factors/mitogens for cell cycle progression and complete the rest of the steps without their need. The point after which the cell cycle progression becomes independent of growth factors/mitogens is called the restriction point (R). 
  • Once the dividing cells pass through the restriction point, cell cycle progression is no longer dependent on growth factors/mitogens. 
  • The concept of restriction point is based on classical experiments on cell culture where the withdrawal of growth factors/mitogens from the culture arrested cells in the G1 phase. However, after crossing a certain point, the withdrawal of growth factors/mitogens no longer was able to inhibit cell cycle progression.
  • Restriction point is critical to cell cycle progression as the decision “to divide” or “not to divide” is taken by the cell. A deregulated restriction point has been observed in most tumors.

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