Role in replication licensing The replication licensing system acts to ensure that the no section of the genome is replicated more than once in a single cell cycle. The inactivation of any of at least five of the six MCM subunits during S phase quickly blocks ongoing elongation. As a critical mechanism to ensure only a single round of DNA replication, the loading of additional MCM2-7 complexes into pre-RCs is inactivated by redundant means after passage into S phase. MCM2-7 activity can also be regulated during elongation. The loss of replication fork integrity, an event precipitated by DNA damage, unusual DNA sequence, or insufficient deoxyribonucleotide precursors, can lead to the formation of DNA double-strand breaks and chromosome rearrangements. Normally, these replication problems trigger an S-phase checkpoint that minimizes genomic damage by blocking further elongation and physically stabilizing protein-DNA associations at the replication fork until the problem is fixed. This stabilization of the replication fork requires the physical interaction of MCM2-7 with Mrc1, Tof1, and Csm3 (M/T/C complex). In the absence of these proteins, dsDNA unwinding and replisome movement powered by MCM2-7 continue, but DNA synthesis stops. At least part of this stop is due to the dissociation of polymerase ε from the replication fork. Biochemical structure Each subunit in the MCM structure contains two large N- and C-terminal domains. The N-terminal domain consists of three small sub-domains and appears to be used mainly for structural organization. The N-domain can coordinate with a neighboring subunit's C-terminal AAA+ helicase domain through a long and conserved loop. This conserved loop, named the allosteric control loop, has been shown to play a role in regulating interactions between N- and C-terminal regions by facilitating communication between the domains in response to ATP hydrolysis [10]. The N-domain also establishes the in vitro 3′→5′ directionality of MCM. Models of DNA unwinding Regarding the physical mechanism of how a hexameric helicase unwinds DNA, two models have been proposed based on in vivo and in vitro data. In the "steric" model, the helicase tightly translocates along one strand of DNA while physically displacing the complementary strand. In the "pump" model, pairs of hexameric helicases unwind duplex DNA by either twisting it apart or extruding it through channels in the complex. Steric model The steric model hypothesizes that the helicase encircles dsDNA and, after local melting of the duplex DNA at the origin, translocates away from the origin, dragging a