Metazoan lagging-strand DNA synthesis is distributive

DNA's anti-parallel structure presents a topological challenge to the replisome, as leading and lagging strands must be synthesized in opposite directions. How lagging-strand synthesis is coordinated with leading-strand synthesis remains unresolved in eukaryotes. To address this question, we directly visualized the dynamics of lagging-strand polymerases Pol α and Pol δ at active replication forks using single-molecule imaging in Xenopus nuclear extracts. Surprisingly, we find that neither Pol α nor Pol δ is stably tethered to the replisome. Instead, lagging-strand synthesis occurs distributively and entails the rapid recruitment of new molecules of Pol α and Pol δ for each Okazaki fragment. Consequently, leading- and lagging-strand synthesis are not intrinsically coupled. We propose that metazoans solve the problem of anti-parallel replication not by actively coordinating leading- and lagging-strand synthesis, but instead simply by recruiting new polymerases to synthesize each Okazaki fragment.