The growth rate is controlled by the proteins involved in the cell cycle (especially in G1/S and G2/M phases).
At the end of each stage of the cycle, the cell can continue its progression, or stop. The transitions between the different phases are controlled by variations in kinase complex activity (serine and threonine activity). This complex consists of two key proteins, two subunits:
- a catalytic subunit = cdk (cyclin-dependent kinase)
- a regulatory subunit = cycline
The concentration of cdk is constant during the cycle. CDKs are only active (ie, capable of phosphorylating target proteins) when combined with cyclins. The concentration of cyclins varies during the cycle (hence their name!).
Other proteins can influence the rate of division, such as those that control the presence of DNA damage, such as ATM ATR, Chk1, Chk2, P21 and especially P53 factors (which is known by its involvement in several cancers).