Connection 1: Cell Cycle and Cancer

Core concept: Cancer = uncontrolled cell division = breakdown of normal cell cycle regulation Mechanism: Mutations in:

• Proto-oncogenes → become oncogenes (accelerate cycling)
• Tumour suppressor genes (e.g., p53, Rb) → lose brakes on cycling Cell cycle checkpoint failures: G1 checkpoint (most commonly bypassed in cancer) NEET connection: Understand mitosis for growth; cancer = runaway mitosis

Connection 2: Down Syndrome and Non-Disjunction

Core concept: Trisomy 21 (Down syndrome) results from non-disjunction in meiosis I of oogenesis Mechanism: Homologous chromosomes 21 fail to separate in meiosis I → one gamete gets both chromosomes 21 → fertilisation gives 3 copies (trisomy 21) Risk increases with maternal age: Oocytes arrested in diplotene for decades → longer exposure to damaging factors → higher non-disjunction risk

Connection 3: Colchicine and Medicine

Core concept: Colchicine inhibits tubulin polymerisation → metaphase arrest Medical use: Treatment of gout (inhibits microtubule-dependent neutrophil migration) Research use: Karyotyping (arrest cells in metaphase for chromosome counting) Cancer treatment: Vinca alkaloids (vincristine, vinblastine) use the same principle to arrest cancer cell division

Connection 4: G0 and Tissue Regeneration

Core concept: Tissues with more cells in active cycling regenerate faster after injury Liver: Hepatocytes are G0 but can re-enter → liver can regenerate after 70% removal Neurons: Permanently G0 → spinal cord injuries cannot heal → why neurological damage is permanent Skin/gut: Constantly cycling → rapid healing but also higher cancer risk (colon cancer, skin cancer)