Cyclin-dependent kinase that acts as a master regulator of the mitotic and meiotic cell cycles. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylation at Thr-14 or Tyr-15 inactivates the enzyme, while phosphorylation at Thr-181 activates it. Forms a stable but non-covalent complex with a regulatory subunit (SUC1) and with a cyclin. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.