Prokaryotic DNA Replication : Complementary strand separation

In order for the two complementary strands of the parental dsDNA to be replicated, they must first separate (or “melt”) over a small region, because the polymerases use only ssDNA as a template. In prokaryotic organisms, DNA replication begins at a single, unique nucleotide sequence, a site called the origin of replication, or ori (oriC in E. coli), as shown in Figure 1A. [Note: This sequence is referred to as a consensus sequence, because the order of nucleotides is essentially the same at each site.] The ori includes short, AT-rich segments that facilitate melting. In eukaryotes, replication begins at multiple sites along the DNA helix (Fig. 1B).
Having multiple origins of replication provides a mechanism for rapidly replicating the great length of eukaryotic DNA molecules.

Figure 1: Replication of DNA: origins and replication forks. A. Small, circular prokaryotic DNA. B. Long, linear eukaryotic DNA.