FIGURE 2.

Cyclin A depletion activates the spindle assembly checkpoint. A, HeLa cells were transfected with control (upper panel) and cyclin A A1 siRNA (lower panel), synchronized with a single thymidine block and harvested in mitosis. Cells were stained for DNA and α-tubulin, and mitotic cells imaged 24-h post-transfection. B, HeLa cells transfected with either scrambled (N), cyclin A siRNAs A1 or A3 were fixed and stained for DNA and microtubules. The percentage of mitotic cells containing formed bipolar spindles and normal metaphase plates (black bars), or bipolar spindles with lagging chromosome migration (gray bars) was assessed. The data are the average of triplicate determinations counting >100 mitotic cells in each experiment. C, cyclin A knockdown HeLa cells were harvested at 24 h after transfection and stained for MAD2 and DNA. A metaphase and prometaphase cell are shown.