Licensing of replication origins

Abstract

All living organisms need to duplicate their genetic material prior to cell division in order to maintain genomic-stability. Cells have evolved sophisticated DNA replication mechanisms to ensure that this process is as faithful as possible. Eukaryotic initiation of DNA replication is a two-step process, where the replicative DNA helicase becomes loaded onto DNA to license DNA replication during late M-phase of the cell cycle prior to helicase-activation in S-phase. Importantly, helicase loading is entirely blocked in S-phase, which is a crucial regulatory mechanism that hinders re-replication of DNA and is crucial for genomic stability. Moreover, multiple copies of the replicative helicase become loaded at each origin to serve as backup-helicases in case a fork becomes terminally arrested. For these reasons it is imperative that helicase loading is as efficient as possible. MCM2-7 represent the core of the replicative helicase, which becomes loaded in an ATP-hydrolysis-dependent process as a double-hexamer onto double-stranded DNA. Current data suggest a model where ORC, Cdc6, and Cdt1 load in a stepwise process the MCM2-7 double-hexamer onto DNA. In this review we discuss the emerging mechanism of ATP-hydrolysis-driven helicase loading, the regulation of this process, and the structure and function of the MCM2-7 double-hexamer.