Figure 1. CMG action at origins.

CMG complexes, which appear as stacked rings with separate N- and C-domains, translocate in the N-first direction. Initially, two N-to-N (head-to-head) CMGs (purple and yellow) are assembled surrounding dsDNA (blue lines) at origins (left), and the tight connection of the inactive Mcm2-7 double hexamer is lost (Abid Ali and Costa, 2016; Costa et al., 2014), depicted as a separation between the CMGs (left). Upon addition of Mcm10 (green, middle) both CMGs switch to opposite strands of ssDNA and can pass one another to form bidirectional forks (right).