Cyclin A2 is not required for sister separation at anaphase II but is necessary to avoid lagging chromosomes. (a) Analysis of anaphase II in cyclin A2^−/− oocytes. Cyclin A2^−/− (n = 88) and control (n = 94) oocytes showed similar rates of sister separation (blue + green + orange bars), but the proportion of oocytes undergoing polar body extrusion (PBE) extrusion (blue + green bars) was reduced (57.7% vs. 96.3%, respectively). Lagging chromosomes were significantly increased in cyclin A2^−/− oocytes compared with cyclin A2^fl/fl controls (75.4% vs. 16%, respectively). Color coding matches quantification with phenotype as determined by imaging. The numbers in parentheses represent oocyte numbers. Time stamps on images represent time elapsed after transfer to activation stimulus (7% ethanol). Arrows show lagging chromatids. Bar, 10 µm. ***, P < 0.001 by χ² test. (b) Time from ethanol exposure to anaphase onset is similar in cyclin A2^fl/fl (n = 88) and cyclin A2^−/− (n = 69) oocytes. Error bars represent SEM. P > 0.05 by t test. (c, i and ii) Oocytes that failed to undergo PBE (orange bar) showed highly disrupted MT structures and dispersed individual chromatids. (c, iii) Chromosome spread of cyclin A2^−/− oocyte that failed to extrude the second polar body. The sister chromatids have undergone separation. Bars: (c, i and ii) 10 µm; (c, iii) 2 µm.