Practice question

Placement of contractile ring determining cleavage plane is actively specified by mitotic spindle rather than cell geometry alone. During anaphase, after chromatids separate, antiparallel interpolar microtubules overlapping in middle zone are bundled by PRC1 protein and kinesins KIF4 and MKLP1, forming central spindle enriched with chromosome passenger complex including Aurora B kinase and centralspindlin heterotetramer of MKLP1-MgcRacGAP. This platform concentrates Rho guanine nucleotide exchange factor ECT2 at adjacent equatorial cortex, converting inactive RhoA-GDP to active RhoA-GTP in narrow band precisely midway between segregated genomes. Active RhoA stimulates formin mDia1 to nucleate linear actin filaments and ROCK-mediated phosphorylation of myosin regulatory light chain driving myosin II bipolar filament assembly and constriction. Astral microtubules radiating to polar cortex deliver inhibitory signals via MP-GAP suppressing Rac-mediated branched actin and Aurora A mediated inhibition, ensuring furrow ingresses only at equator perpendicular to central spindle, guaranteeing equal genome partitioning between daughters. 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.