Cyclin B1^−/− embryos arrest in G2 phase after completing S phase. (A) Progress through S phase in the early mouse embryo can be monitored by injection of RFP-PCNA mRNA. The characteristic patterns of PCNA foci during S phase, and its homogenous distribution in G1 and G2 phases, were used to analyze the timing of different cell cycle phases in the early embryo. Bar, 10 µm. (B) Zygotes were injected with RFP-PCNA and representative frames of the Cyclin B1^−/− embryo in Video 9 are shown from the two-cell stage until the corresponding wild-type reaches the blastocyst stage (bar, 20 µm). Enlarged inserts on top shows how one nucleus progresses normally through S phase after the second division and then arrests with homogenous PCNA distribution indicative of G2 phase (n = 5 embryos in three independent experiments). Bar, 10 µm (insets). (C) Representative frames of the RFP channel in Video 7. One blastomere was injected with H2B-RFP mRNA at the two-cell stage. Wee1 inhibitor was added to arrested Cyclin B1–null embryos at the time when wild-type embryos had started the four- to eight-cell division. Inhibitor was added during first time point. Wild-type (top) and Cyclin B1^−/− (middle) chromatin condensation and some dispersal indicating NEBD (1 of 13 embryos); Cyclin B1^−/− (bottom): chromatin condensation but no dispersal. Bar, 20 µm; time in hours:minutes.