Figure 4.

Model of how cyclin B3 may promote anaphase I onset in mouse oocytes. Cyclin B1/B2-Cdk1 (in blue) activity increases as oocytes progress through prometaphase into metaphase, because the APC/C (light blue) in association with its activator Cdc20 (brown) is kept in check by the SAC. At the metaphase-to-anaphase transition, the SAC is satisfied and inactivated, allowing full APC/C activity, ubiquitination and hence degradation of cyclin B1 and B2 in association with Cdk1. In oocytes, full APC/C activity requires the function of cyclin B3-Cdk1. Cyclin B3-Cdk1 (in green) promotes APC/C activity leading to chromosome segregation and exit from meiosis I. Therefore cyclin B3, a late substrate of the APC/C, shows an opposing role to cyclin B1 and B2 during mouse oocyte meiosis I.