In 2 places
1) At the point of entry into cell cycle when growth factors are available (=late G1 aka the restriction point)….but if there is no growth factor it enters quiescence stage (=G0) aka quiet/no div
2) At several critical checkpoints (this actions is exerted through enzyme cyclin dependent kinase cdks)
simple —> Cdks is the main controller that does the checking at the checkpoint)
(for better understanding oo!) Cdks role? is simply to ensure that every part of the cell is in order before moving to the next stage :).
Phosphorylate to add negative charge.