Acts as a negative regulator of entry into mitosis (G2 to M transition) by phosphorylation of the CDK1 kinase during oocyte maturation (PubMed:16338136, PubMed:16421191, PubMed:16466390). Required for oocyte maturation, embryonic development, germline proliferation and initiation of meiosis during spermatogenesis (PubMed:11702779, PubMed:12397109, PubMed:27104746). Required for chromosome structure during mitosis and negative regulation of nuclear envelope breakdown (PubMed:16421191). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed during both germline and early embryonic development. In larvae, it is expressed in the distal region, germline, and in some neurons and hypodermal cells. Not expressed in distal region of the adult. Worms exhibit oocytes that mature prematurely (also known as the Emo phenotype) and chromosomes that appear condensed (PubMed:16421191). Subcellular defects include irregular perinuclear foci in oocytes (PubMed:16421191). RNAi-mediated knockdown results in an accumulation of damaged, compressed, polyploid oocytes in the uterus (PubMed:11702779). RNAi-mediated knockdown in a double oma-1 and oma-2 knockout background allows for the progression of meiosis beyond the prophase stage and oocyte maturation, but these oocytes cannot be fertilized (PubMed:11702779). RNAi-mediated knockdown in a cdc-25.2 mutant background suppresses the defect in intestinal cell divisions in the cdc-25.2 single mutant (PubMed:27104746). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.