Definition of the Cell Cycle Segment with Special Sensitivity to Vinblastine

The ability of Vb to produce multinucleation in HeLa cells was used to define a period of the cell cycle with special sensitivity to vinblastine. Exponentially growing monolayers of HeLa cells were treated with Vb for 30 min. After removal of Vb, mitotic cells were collected, seeded on coverslips, and examined microscopically. Maximal multinucleation was found when the cells were treated with Vb about 5 hr before mitosis, the period of time which corresponds to the late S, close to the S-G2 boundary of these cells. Simultaneous exposure to Vb and [³H]thymidine showed that DNA synthesis was taking place during the period of maximal sensitivity to Vb, and treatment and inhibitors of DNA synthesis abolished this Vb effect. These findings indicate that events take place near the end of the S phase which affect the integrity of the spindle at the subsequent mitosis.