Fig. 6.

Staircase model of hexameric helicase action. In the staircase model of DNA unwinding, the helicase encircles one strand as a staircase, and the subunit at the bottom of the spiral staircase (i.e., subunit A is shown in red) hydrolyzes ATP and then moves to the top of the spiral and exchanges ADP for ATP to rebind DNA. This process is proposed to repeat around the ring to move the staircase along the ssDNA. Studies of CMG as a staircase indicate unique properties of each ATP site (10) as do studies of individual ATP sites in reconstitution studies (35, 36).