Abstract

Significance All cellular life forms use a ring-shaped hexameric helicase during DNA replication. CMG (Cdc45, Mcm2–7, GINS) is the eukaryotic replicative helicase. CMG contains the ring-shaped hexameric Mcm2–7 that harbors the helicase motors. CMG is known to bind many other proteins, including a leading and lagging polymerase and primase. Thus, the threading of DNA through the CMG helicase at a replication fork determines the orientation of the associated polymerases at the replication fork, an important structural feature with many consequences that may direct future experimentation. This report uses cryo-EM single-particle reconstruction to image CMG that motored to a block site at a forked junction, enabling direct visualization of DNA threading through CMG.