Practice question

Animal cytokinesis culminates mitosis by physically partitioning cytoplasm through contractile ring machinery. Following chromosome segregation, central spindle composed of overlapping antiparallel microtubules bundled by PRC1 and motor KIF4 recruits centralspindlin complex containing kinesin MKLP1 and RhoGAP MgcRacGAP which tethers ECT2 guanine nucleotide exchange factor to equatorial cortex, generating localized accumulation of active RhoA-GTP. RhoA stimulates formin mDia1 to nucleate linear actin filaments and activates ROCK kinase to phosphorylate myosin regulatory light chain at serine 19, promoting assembly of bipolar myosin II filaments into antiparallel array that slides on actin similarly to muscle contraction, decreasing ring diameter. Scaffold proteins anillin, containing actin, myosin, and membrane lipid binding domains, and septins stabilize ring position. Ingression continues until midbody forms with bundled microtubules. ESCRT-III complex comprising CHMP4B, CHMP2A, and VPS4 ATPase executes final membrane abscission. Inhibitors latrunculin or blebbistatin block division producing multinucleated cells, proving actin-myosin dependence. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.