The MCM2-7 complex is believed to function as the eukaryotic replicative DNA 
helicase. It is recruited to chromatin by the origin recognition complex (ORC), 
Cdc6, and Cdt1, and it is activated at the G(1)/S transition by Cdc45 and the 
protein kinases Cdc7 and Cdk2. Paradoxically, the number of chromatin-bound MCM 
complexes greatly exceeds the number of bound ORC complexes. To understand how 
the high MCM2-7:ORC ratio comes about, we examined the binding of these proteins 
to immobilized linear DNA fragments in Xenopus egg extracts. The minimum length 
of DNA required to recruit ORC and MCM2-7 was approximately 80 bp, and the 
MCM2-7:ORC ratio on this fragment was approximately 1:1. With longer DNA 
fragments, the MCM2-7:ORC ratio increased dramatically, indicating that MCM 
complexes normally become distributed over a large region of DNA surrounding 
ORC. Only a small subset of the chromatin-bound MCM2-7 complexes recruited Cdc45 
at the onset of DNA replication, and unlike Cdc45, MCM2-7 was not limiting for 
DNA replication. However, all the chromatin-bound MCM complexes may be 
functional, because they were phosphorylated in a Cdc7-dependent fashion, and 
because they could be induced to support Cdk2-dependent Cdc45 loading. The data 
suggest that in Xenopus egg extracts, origins of replication contain multiple, 
distributed, initiation-competent MCM2-7 complexes.