01Hitting dividing cells
Classic chemotherapy targets processes all dividing cells need. Alkylating agents and platinum drugs (cisplatin) cross-link DNA. Antimetabolites (5-FU, methotrexate) starve DNA synthesis. Topoisomerase inhibitors (doxorubicin, etoposide) trap DNA breaks. Microtubule drugs (taxanes, vincristine) jam mitosis.
Side effects follow logically: hair follicles, gut lining and bone marrow divide quickly too, causing hair loss, nausea and low blood counts.
02Radiation
Ionizing radiation causes double-strand breaks, mostly through free radicals. Doses are split into many small fractions so normal tissue can repair between sessions while tumor cells — often with broken checkpoints — accumulate lethal damage.
Hypoxic tumor regions resist radiation because oxygen helps 'fix' the damage permanently.