Cryo-electron microscopy (cryo-EM) imaging of DNA replication origin activation explains the role of Mcm10, a minichromosome maintenance (MCM) protein homolog, during initiation. Mcm10 acts as a wedge to split the two MCM hexamers of the activated replicative helicase. Diverging replication forks are then established, with changes in the MCM hexamers that promote the topological separation of two DNA strands.