Injection of Geminin during the Drosophila blastoderm divisions deletes S phase and advances mitosis. (A) Schematic illustrating progression through the blastoderm divisions and the MBT. The bar beneath the graph represents time. Cdc25^stg protein declines during the blastoderm cycles. Zygotic transcription of Cdc25^stg during G2 of cycle 14 promotes mitosis. (B) Schematic illustrating DNA replication initiation and experimental expectation from Geminin injection. (C) Nuclei of YFP-PCNA–expressing embryos in the cycle after injection of Geminin or control buffer. YFP-PCNA exhibits distinct nuclear foci in control injections. Arrows highlight foci in the apical region of nuclei at the end of S phase. Note that YFP-PCNA foci are absent throughout S phase after Geminin injection. Frames 9:56 and 11:48 of the Geminin-injected embryo correspond to premature mitotic entry and dispersal of nuclear YFP-PCNA in the cytoplasm. (D) YFP-PCNA–expressing embryos were injected in cycle 11 or cycle 12 with Geminin and interphase length and mitosis were measured in the subsequent cycle (cycle 12 or cycle 13, respectively). Interphase was defined by nuclear localization of PCNA and mitosis was defined by its dispersal. Average cell cycle times are shown for cycle 12 (n = 7 embryos) and cycle 13 (n = 10 embryos). Error bars represent SD.