A 56-year-old woman is recently diagnosed with breast cancer. She undergoes a lumpectomy and a lymph node dissection. The biopsy results indicate that tumor cells have migrated to the lymph nodes. Her oncologist recommends chemotherapy including the agent paclitaxel (Taxol), which blocks microtubule depolymerization, thereby interfering with the cells’ ability to traverse the cell cycle. During which phase of mitosis and the cell cycle would this drug be most active?