Mcm2-7 is a ring-shaped DNA helicase that plays an essential role in DNA repliction in eukaryotic cells.  Two of the helicase molecules must encircle the double-stranded DNA at a replication origin, establishing a loaded, anti-parallel double-ring complex able to start replication at the appropriate cell cycle stage.  In this study, Simina Ticau together with collaborators from Steve Bell’s lab (MIT), Jeff Gelles’ lab (Brandeis), and New England BioLabs used wild-type and mutant helicases in single-molecule colocalization (“CoSMoS”) and single-molecule fluorescence resonance energy transfer (smFRET) experiments to identify the mechanisms by which regulatory factors and nucleotide hydrolysis control ring opening and coordinate loading. This work reveals the molecular processes that serve to prevent catastrophic genome damage due to incorrect or mistimed assembly of the replicative machinery.