8.6:
Replication in Prokaryotes
In prokaryotes, such as E. coli, DNA replication of the circular, double–stranded genome begins at the origin of replication called OriC.
Initiator proteins bind to OriC and initiate DNA separation. Then, a helicase is loaded onto each strand to unwind the DNA further and form a replication bubble.
The unwound DNA is stabilized by single-strand DNA binding proteins or SSBs.
The replication bubble now has two replication forks on either side that proceed bidirectionally.
At each fork, the multi-protein replication machinery, including DNA polymerases, simultaneously synthesize the leading strand and the lagging strand.
As the unwinding continues, the DNA ahead of the fork becomes overwound and supercoiled, creating a torsional strain.
Type one topoisomerase enzymes help relieve this stress by creating a nick to allow the other strand to pass through and then ligating the DNA back.
As the replication continues along the genome, the two forks meet at the termination or ter sites where DNA replication completes.
The resulting daughter DNA molecules are composed of one parental strand and one newly synthesized strand following the semi-conservative model of DNA replication.
8.6:
Replication in Prokaryotes
DNA replication has three main steps: initiation, elongation, and termination. Replication in prokaryotes begins when initiator proteins bind to the single origin of replication (ori) on the cell's circular chromosome. Replication then proceeds around the entire circle of the chromosome in each direction from the two replication forks, resulting in two DNA molecules.
Many Proteins Work Together to Replicate the Chromosome
Replication is coordinated and carried out by a host of specialized proteins. Topoisomerase breaks one side of the double-stranded DNA phosphate-sugar backbone, allowing the DNA helix to unwind more rapidly, while helicase breaks the bonds between base pairs at the fork, separating the DNA into two template strands. Proteins that bind single-stranded DNA molecules stabilize the strands as the replication fork travels along the chromosome. DNA can only be synthesized in the 5' to 3' direction, so one strand of the template—the leading strand—is elongated continuously, while the other strand—the lagging strand—is synthesized in shorter pieces of 1,000–2,000 base pairs called Okazaki fragments.
Multiple Polymerases Take Part in Elongation
Much of the research to understand prokaryotic DNA replication has been performed in the bacterium Escherichia coli, a commonly-used model organism. E. coli has five DNA polymerases: Pol I, II, III, IV, and V. Pol III is responsible for the majority of DNA replication. It can polymerize about 1,000 base pairs per second. This astonishing pace allows the machinery present at the two replication forks to duplicate the E. coli chromosome—4.6 million base pairs—in roughly 40 minutes. DNA polymerase I is also well-characterized. Its primary role is to process the RNA primers on the lagging strand.
When Division Outpaces Duplication
Under favorable growth conditions, E. coli can divide every 20 minutes, about half the amount of time that it takes to replicate the genome. But how is this possible when both daughter cells must have their own DNA? Scientists found that the bacteria can begin another round of DNA replication from the origin of replication before the first round is complete; this means that daughter cells receive a chromosome that is already in the process of being copied and are prepared to divide again very quickly.