Prematurely increasing CDK1 activity stabilizes K-MT attachments. (A and B) Oocytes injected with GFP cRNA or cyclin B–GFP cRNA (700 ng/µl) were analyzed for histone-H1 kinase activity (A) or cold-stable MTs (B) at the indicated time points. H1 kinase activity (A) was averaged over three lysates, each with one oocyte. Images in B are projections of a confocal z series showing MTs (green) and kinetochores (CREST, red). Insets show individual kinetochores classified as attached (1), unattached (2), or lateral (3); bar, 0.5 µm. Injecting lower levels of cyclin B–GFP cRNA (200 ng/µl) together with CDK1 cRNA gave similar results in both the H1 kinase and cold-stable MT assays. (C and D) Percentages of each attachment state were averaged over multiple cells (n ≥ 22, 15 kinetochores per cell; *, P < 0.001) at each time point. Results of the cold-stable MT assay were combined for oocytes injected with cyclin B–GFP cRNA (700 ng/µl) or CDK1 cRNA together with lower levels cyclin B-GFP cRNA (200 ng/µl).