Resumption of oocyte meiotic maturation is driven by activation of cyclin-dependent kinase 1 (Cdk1), which
phosphorylates target proteins that are important for nuclear envelope breakdown, chromosome condensation, and remodelling of the actin and microtubule cytoskeleton. As a consequence, a bipolar spindle is formed and condensed chromosomes become aligned at the metaphase plate. The spindle assembly checkpoint (SAC) prevents the activation of the anaphase-promoting complex/cyclosome (APC/CCdc20) during meiosis I progression until all kinetochores come under tension [for review see: 1]. Upon the satisfaction of SAC, APC/C becomes active, and degrades cyclin B and securin. Degradation of cyclin B lowers Cdk1 activity and securin degradation activates separase, which cleaves cohesion holding sister chromatids together . After cytokinesis, one set of chromosomes is retained in the oocyte and the other is segregated into the first polar body (PB).