Practice question

Myosin II is the principal motor generating contractile force for cell division in animal cells. It assembles into bipolar thick filaments through coiled coil tail dimerization, each filament containing about 30 dimers with heads oriented opposite at ends. In anaphase, equatorial cortex enriched in active RhoA GTP recruits anillin, septins and centralspindlin, concentrating myosin II and actin into contractile ring. Phosphorylation of regulatory light chain by ROCK and citron kinase activates myosin II ATPase increasing ten fold, driving filament sliding that constricts ring from 30 micrometers to midbody during cytokinesis. Ring contraction coupled with plasma membrane insertion via recycling endosomes and ESCRT III mediated abscission completes separation. Myosin II does not transport organelles which requires myosin V, does not assemble microtubules nor segregate chromosomes which depend on microtubule kinesin dynein. Genetic depletion or blebbistatin inhibition blocking myosin II ATPase causes multinucleation confirming cytokinesis role. This contractile mechanism ensures equal partition of cytoplasm and maintenance of genomic stability across daughter cells during mitotic exit.