RESTRICTION POINT
Restriction point (point R) is the most sensitive point of stage G1 of the cell cycle. It lies closer to the end of stage G1. Cells usually stop dividing after attaining the restriction point, if only they do not receive the signal, which causes them to enter next cycle. When conditions are unfavourable (for example, when cell density increases, or, when cells are starving), in the restriction point cell growth inhibition occurs. The cells, which were stopped in this stable stage, are said to enter into stage G0 of the cell cycle. G0-period is called resting period. Mammalian cells have a specific regulatory system with participation of a special trigger protein. This protein was named like this because it works on the trigger principle. The regulatory system re strains cell cycle in the restriction point as long as all components, which are necessary for completion of all sequential processes of assembling of complicated supermolecular structures in the forthcoming stages S, G2 and M, are synthesized.