(A) SSB (single strand binding protein) holds denatured oriC open. DnaB helicase has been loaded onto replication forks as detailed in Figure H3.
(B) DnaG primase is recruited to the replication fork by protein-protein interactions with DnaB helicase.
Note: Going from (B) to (C) to (D), it is clear that DnaB helicase moves in the 5'-to-3' direction along DNA strands.
(C) Primase synthesizes short (about 10 nucleotides long) RNA molecules that will act as primers for DNA synthesis.
(D) Primase requires contact with SSB for tight binding to its primed site and must be displaced so that the beta clamp can be loaded onto DNA. The clamp loader (gamma complex) of DNA polymerase III holoenzyme puts a beta clamp around an RNA primer using ATP energy. A primer-terminus (3' hydroxyl of the RNA primer) is also required in order for the clamp loader to place a beta ring onto the RNA-DNA structure. The beta ring is constrained to the primer since it cannot slide over single-stranded DNA. The chi subunit of the clamp loader is also the primase-displacing subunit. The underlying mechanism is a competitive contact between chi and primase for SSB. Upon establishing the chi-to-SSB contact, the primase-to-SSB contact is disrupted and primase diffuses away allowing gamma complex to assemble beta onto the primed site.
(E) This allows the attachment of the alpha polymerase subunit to the beta clamp and positions the 3'-hydroxyl end of the RNA primer in DNA polymerization active site so that DNA synthesis can begin. Hence, the primase-to-polymerase switch is orchestrated by an ordered set of reactions at a primed site in which SSB makes mutually exclusive contacts with primase and the chi subunit of the gamma complex needed to load beta. Since beta is needed to tether core polymerase to DNA, the end result is a primase-to-polymerase switch.
(F) Tethered DNA polymerase III now synthesizes the leading strand. DNA synthesis proceeds at a rate of 1,000 nucleotides per second. Protein-protein contact between DNA polymerase III and DnaB helicase stimulates helicase activity.