Structures of the simplest replisome The DNA replisome performs concerted parental-strand separation and DNA synthesis on both strands. Gao et al. report the cryo–electron microscopy structures of the minimum set of bacteriophage T7 proteins that can carry out leading- and lagging-strand synthesis at the replication fork (see the Perspective by Li and O'Donnell). Three key enzymes involved in DNA replication—DNA polymerase, helicase, and primase—were visualized in complex with substrate DNA, demonstrating their highly dynamic organizations on both strands. Comparison of prokaryotic and eukaryotic replisomes reveals evolutionarily conserved operating principles and provides a structural basis for understanding coordination among DNA replication, recombination, and repair. Science , this issue p. eaav7003 ; see also p. 814