ParM is a prokaryotic actin homologue which provides the force to drive copies of the R1 plasmid to opposite ends of rod shaped bacteria before cytokinesis. ParM is a monomer that is encoded in the DNA of the R1 plasmid and manufactured by the host cell’s ribosomes. In the cytoplasm it spontaneously polymerizes forming short strands that either bind to ParR or hydrolyze. ParR stabilizes ParM and prevents it from hydrolyzing. Once bound by ParR at both ends, monomer units continue to attach to the ends of the ParM and the resulting reaction pushes R1 plasmids to opposite ends of the cell.ParMs from different bacterial plasmids can form astonishingly diverse helical structures comprising two or four strands to maintain faithful plasmid inheritance.