Explanation

The cell cycle is controlled by enzymes like cyclin dependent kinases (CdKs). CdKs phosphorylate amino acids like serine and threonine which initiates or blocks the activities related to cell cycle. The other check points involved in cell cycle are $G_{1}$check point (Enter S or synthesis) is controlled by $CdK_{4}$/Cyclin D, $CdK_{6}$/Cyclin D $G_{2}$ check point (Enter M or maturation promoting factor) by is controlled $CdK_{2}$/cyclin B Metaphase check point is controlled by cyclin B degradation