A model of the replication fork proposed by Bruce Alberts thirty years ago provides an elegant solution for coupling of leading- and lagging-strand synthesis. Here we present the first structure of a megadalton-sized functional replisome of bacteriophage T7 assembled on DNA resembling a replication fork. Structure of the complex consisting of DNA helicase, RNA primase, and two DNA polymerase molecules was determined by single-particle cryo-electron microscopy. The two molecules of DNA polymerase adopt different spatial arrangement at the replication fork reflecting their roles in leading- and lagging-strand synthesis. The structure reveals molecular mechanisms for coordination of leading- and lagging-strand synthesis, recycling of DNA polymerases, formation and release of the lagging-strand replication loop, and stabilization of the RNA primer on the lagging-strand for subsequent extension to an Okazaki fragment. Since mechanisms of DNA replication are highly conserved, the observations are relevant to other replication systems.