 Again, it is the interaction between the resolvase dimers bound at accessory sites (sites II and III) and resolvase at site I that causes the two dimers to synapse and form a tetramer.After the tetramer is formed it becomes activated and the top and bottom DNA strands are simultaneously cleaved in the middle of the site I with a 2bp overhang.The strand exchange ensues by as yet unknown mechanism with a resulting net rotation of 180°.