Acts as a negative regulator of entry into mitosis (G2 to M transition) by phosphorylation of the CDK1 kinase during oocyte maturation. Required for embryonic development, germline proliferation and initiation of meiosis during spermatogenesis. Required for chromosome structure during mitosis and negative regulation of nuclear envelope breakdown (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.