<P> The assembly of the minichromosome maintenance (Mcm) proteins function together as a complex in the cell . The assembly of the Mcm proteins onto chromatin requires the coordinated function of the origin recognition complex (ORC), Cdc6, and Cdt1 . Once the Mcm proteins have been loaded onto the chromatin, ORC and Cdc6 can be removed from the chromatin without preventing subsequent DNA replication . This suggests that the primary role of the pre-replication complex is to correctly load the Mcm proteins . </P> <P> The Mcm proteins support roles both in the initiation and elongation steps of DNA synthesis . Each Mcm protein is highly related to all others, but unique sequences distinguishing each of the subunit types are conserved across eukaryotes . All eukaryotes have exactly six Mcm protein analogs that each fall into one of the existing classes (Mcm2 - 7), which suggests that each Mcm protein has a unique and important function . </P> <P> Minichromosome maintenance proteins have been found to be required for DNA helicase activity and inactivation of any of the six Mcm proteins prevents further progression of the replication fork . This is consistent with the requirement of ORC, Cdc6, and Cdt1 function to assemble the Mcm proteins at the origin of replication . The complex containing all six Mcm proteins creates a hexameric, doughnut like structure with a central cavity . The helicase activity of the Mcm protein complex raises the question of how the ring - like complex is loaded onto the single - stranded DNA . One possibility is that the helicase activity of the Mcm protein complex can oscillate between an open and a closed ring formation to allow single - stranded DNA loading . </P> <P> Along with the minichromosome maintenance protein complex helicase activity, the complex also has associated ATPase activity . A mutation in any one of the six Mcm proteins reduces the conserved ATP binding sites, which indicates that ATP hydrolysis is a coordinated event involving all six subunits of the Mcm complex . Studies have shown that within the Mcm protein complex are specific catalytic pairs of Mcm proteins that function together to coordinate ATP hydrolysis . For example, Mcm3 but not Mcm6 can activate Mcm6 activity . These studies suggest that the structure for the Mcm complex is a hexamer with Mcm3 next to Mcm7, Mcm2 next to Mcm6, and Mcm4 next to Mcm5 . Both members of the catalytic pair contribute to the conformation that allows ATP binding and hydrolysis and the mixture of active and inactive subunits create a coordinated ATPase activity that allows the Mcm protein complex to complete ATP binding and hydrolysis as a whole . </P>

Process of dna replication in prokaryotes and eukaryotes