Figure 1.

Activity states of the replicative DNA helicase at bacterial and eukaryotic replication origins. In bacteria the replicative DNA helicase is loaded in active form around single-stranded DNA at the origin (left). In eukaryotes, the replicative DNA helicase is loaded in inactive form around double-stranded DNA at the origin; activation of the helicase is temporally separated from helicase loading in the cell cycle (right). The initiator protein(s) direct the loading of the replicative DNA helicase. G1 and S indicate the G1 phase and S phase cell cycle stages in eukaryotes.