Chromatin Constrains the Initiation and Elongation of DNA Replication

Eukaryotic chromosomal DNA is faithfully replicated in a complex series of cell-cycle-regulated events that are incompletely understood. Here we report the reconstitution of DNA replication free in solution with purified proteins from the budding yeast Saccharomyces cerevisiae. The system recapitulates regulated bidirectional origin activation; synthesis of leading and lagging strands by the three replicative DNA polymerases Pol alpha, Pol delta, and Pol epsilon; and canonical maturation of Okazaki fragments into continuous daughter strands. We uncover a dual regulatory role for chromatin during DNA replication: promoting origin dependence and determining Okazaki fragment length by restricting Pol delta progression. This system thus provides a functional platform for the detailed mechanistic analysis of eukaryotic chromosome replication.