Practice question

Cytokinesis in animal cells determines placement of furrow such that daughter cells inherit single genome copy and appropriate cytoplasmic volume. Experimental embryology demonstrated furrow forms where central spindle contacts cortex. Mechanistically, anaphase central spindle assembled from overlapping antiparallel interpolar microtubules bundled by PRC1 and centralspindlin component KIF4 recruits centralspindlin heterotetramer consisting of kinesin MKLP1 and RhoGAP MgcRacGAP. Centralspindlin clusters ECT2 RhoGEF, generating zone of active RhoA-GTP precisely at equatorial membrane. RhoA-GTP activates formin mDia1 to polymerize linear actin filaments and ROCK kinase phosphorylating myosin regulatory light chain serine 19 promoting bipolar myosin II assembly in antiparallel bundles contracting ring similarly to muscle. Astral microtubules reaching polar cortex deliver inhibitory cues through Aurora A phosphorylating ECT2 and RacGEF Trio suppressing ectopic contractility at poles. Net result furrow ingresses perpendicular to central spindle axis between chromosome masses. Manipulation shifting spindle relocates furrow accordingly, proving spindle dominance over chromosomes for division site. 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.