Abstract

During each cell cycle, eukaryotic cells must faithfully replicate their genome, ensuring exactly one full copy is made to maintain cell ploidy. The major mechanism to ensure once per cell cycle genomic replication involves the temporal separation of two key replication events: origin licensing and helicase activation. We have developed an array of single-molecule biochemical assays to reveal the kinetics and mechanism of these processes. I will focus on origin licensing, and discuss the remarkably dynamic events required to assemble the head-to-head dimer of the Mcm2 7 replicative helicase encircling the double-stranded DNA associated with the licensed origin. In particular, I will discuss the role of DNA bending and sliding in the early stages of loading, how studies of the regulation of origin licensing have provided insights into the mechanism of Mcm2-7 ring closing, and evidence that one protein “leap-frogs” another to drive loading of the two helicases in opposite orientations. Together, these studies reveal a mechanism of origin licensing that ensures two helicases are loaded in opposite orientation and the power of single-molecule studies to uncover the complexities of biological processes