Fig 3. Precocious Chk1 activity and cell cycle elongation are not sufficient for SAC acquisition.

(A) Schematic of cell cycle lengths for the first 10 cleavage divisions of embryos injected with Alexa 647-Histone H1, with or without Chk1-4E-GFP mRNA. Cell cycles lengths were measured as time between metaphases, starting from 1.5 HPF, based on Histone H1 morphology. Embryos injected with H1-647 alone (control) had consistent cleavage divisions that each lasted ~16 min as expected [5] while embryos injected with Chk1-4E mRNA and H1-647 had cleavage cycles that progressively lengthened. The measured cell cycle lengths for cleavages 5–10 are indicated by the length of each bar in the schematic and shown in the table (n >4 embryos; error indicates S.D.) (B) Embryos were injected with Alexa 647-Histone H1 and PCNA-GFP proteins and Chk1-4E mRNA, treated with our without nocodazole at 2.25 HPF, then imaged live until 3 HPF. Images show cell cycle progression based on Histone H1 and PCNA. Insets are displayed with higher contrast settings to show nuclear morphology as in Fig. 1. In the montage, time between metaphase and the next interphase is 10 min without nocodazole and 12 min for nocodazole-treated embryos. There was no measurable progressive lengthening of mitoses during the cleavage divisions, regardless of drug treatment. Graph shows average length of mitosis for each condition (n≥12 for each condition, from three independent experiments). Error bars indicate S.D.; P>0.05; two-tailed Student’s t test; scale bars 20 μm.