Final answer:
Chemotherapy drugs disrupt mitosis by interfering with microtubule assembly and disassembly. When cells in mitosis are treated with a drug that depolymerizes microtubules, cell division is interrupted and the Mad2 protein is present in the cytoplasm instead of on the kinetochores of chromosomes.
Step-by-step explanation:
Chemotherapy drugs such as vincristine and colchicines disrupt mitosis by binding to tubulin, the subunit of microtubules, and interfering with microtubule assembly and disassembly. When cells that have started mitosis are treated with nocodazole, a drug that depolymerizes microtubules, it would prevent the formation of mitotic spindles. This would lead to the interruption of cell division. The Mad2 protein is normally localized on the kinetochores of chromosomes during mitosis. However, when cells are treated with nocodazole and the microtubules are depolymerized, the Mad2 protein would not be able to bind to the kinetochores and would be present in the cytoplasm.