DNA replication onset requires the orchestrated action of multiple enzymes that often function as large, modular assemblies. To describe the architecture and dynamics of the eukaryotic DNA replication machinery, we combine biochemistry and single-particle cryo-electron microscopy to elucidate the mechanics of replication origin activation and fork progression. Using these tools we have recently gained important novel insights into the mechanism of DNA translocation by the replicative helicase, CMG. We have also described the physical link between the CMG helicase and the Polymerase alpha component of the lagging strand primosome, starting to explain how DNA unwinding and synthesis are coupled during genome duplication. Our results help establish a molecular framework to explain how eukaryotes respond to DNA damage and how cell proliferation is regulated to avoid tumorigenesis.