MCM double-hexamer formation follows a sequential, quasi-symmetric mechanism.
A first ORC origin-association event involves an ACS, high-affinity DNA site. ORC binds and bends DNA, to allow for the recruitment of a first MCM ring, through a C-terminal MCM interaction. Upon release of ORC from the ACS site, a second ORC-binding event involves the N-terminal MCM domain and an inverted lower-affinity ORC-binding site. In this configuration, ORC is competent for recruiting a second MCM ring, following the same mechanism as the loading of the first MCM ring. The end result of the helicase loading reaction is the formation of a head-to-head double hexameric ring. The use of the ReconSil in silico reconstitution approach allows for the visualisation of a nucleosome, which flanks the ACS site in the reconstituted system and in yeast origins of replication.