Figure 1. Schematic illustration of the roles of Cdk1 during the G₂-M transition in mitosis and in the germinal vesicle breakdown that occurs during resumption of meiosis. (A) Cdk1 kinase is activated and promotes the phosphorylation of mitotic phosphoproteins at the G₂-M transition in mitosis. In order to maintain the phosphorylation status of the Cdk1 substrates, PP2A is inactivated by activation of Gwl by Cdk1. Active Gwl inhibits PP2A through the activation of Arpp19 and Ensa. (B) During the resumption of meiosis in oocytes, Cdk1 is activated and promotes the phosphorylation of meiotic phosphoproteins. Maintenance of the phosphorylation status of the meiotic phosphoproteins is ensured by phosphorylation and inhibition of PP1 by Cdk1. P, phosphorylation; Arpp19, cAMP-regulated phosphoprotein 19; Ensa, α-endosulfine.