Plays a key role in the control of the eukaryotic cell cycle (PubMed:2120044, PubMed:15581871). It is required in higher cells for entry into S-phase and mitosis (PubMed:2120044, PubMed:15581871, PubMed:29746464). In embryos, promotes the release of Rif1 from chromatin during mid-blastula transition (PubMed:29746464). p34 is a component of the kinase complex that phosphorylates the repetitive C-terminus of RNA polymerase II (PubMed:2120044). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] Phosphorylation at Thr-14 or Tyr-15 inactivates the enzyme, while phosphorylation at Thr-161 activates it. Forms a stable but non-covalent complex with a regulatory subunit and with a cyclin. Component of the Frs-CycA-Cdk1 complex composed of Cdk1, CycA and Z600 (PubMed:17431409). Expressed both maternally and zygotically (PubMed:2120044, PubMed:8405984). High levels of expression when mitosis is elevated; highest levels of expression are in early embryos with levels decreasing during embryogenesis and remaining low throughout most of larval development, and expression levels are also increased in unfertilized eggs and adult females (PubMed:2120044). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.