The licensing of eukaryotic DNA replication origins, which ensures once per cell cycle replication, involves the loading of six related minichromosome maintenance proteins (Mcm2-7) into prereplicative complexes (pre-RCs). Mcm2-7 forms the core of the replicative DNA helicase, which is inactive in the pre-RC. The ATP-dependent Mcm2-7 loading reaction requires the Origin Recognition Complex (ORC), Cdc6 and Cdt1. We have reconstituted Mcm2-7 loading with purified budding yeast proteins. Using biochemical approaches and electron microscopy, we show that single heptamers of Cdt1·Mcm2-7 are loaded cooperatively into stable, head-to-head Mcm2-7 double hexamers connected via N-terminal rings. DNA runs through a central channel in the double hexamer, and, once loaded, Mcm2-7 can slide passively along double-stranded DNA. Our work has significant implications for understanding how eukaryotic DNA replication origins are chosen and licensed, how replisomes assemble during initiation and how unwinding occurs during DNA replication.