Initiation of Replication

oriC:In E. coli replication initiates at a locus called oriC which is just a few hundred nucleotides in length. The main components are 3 copies of a 9-base sequence (9-mer) and 3 copies of a 13-base sequence (13-mer).

Formation of the pre-priming complex: About 20 copies of the DnaA protein bind to the 9-mer repeats of oriC. This complex results in a bending of the DNA that loosens the stability of the double helix. This results in the strands separating in the section of three 13-mers, forming what can be described as a "bubble" in the DNA.

At each end of the "bubble" a protein complex consisting of 6 copies each of DnaB and DnaC recognizes the opened complex, with the assistance of the DnaT protein. The 6 DnaB proteins of the complex bind to the DNA and the DnaC is released. The two strands of DNA fully separate at the site of the bend, a process that is assisted by DnaB which has helicase activity. This is the pre-priming complex. The DnaB that is bound will be part of the replication apparatus at the two growing forks that expand bidirectionally from the origin (one in each direction).

Priming: With the two strands now separated they can be used as templates. Since replication is bidirectional four primers need to be synthesized - one for each of the two strands in both directions. DNA primase binds to DnaB, primers are synthesized and DNA PolIII binds at each fork.

As the primers are extended we get into elongation.