The rolling circle model of DNA replication was proposed in 1968. This model explains mechanism of DNA replication in circular plasmids and single stranded circular DNA of viruses.
For many plasmids replication is not tied to chromosomal replication. Many circularly closed plasmids replicate autonomously; by a method called rolling circle replication. Rolling circle replication is also called unidirectional replication.
The machanism of Rolling circle replication
In rolling circle replication a replication initiator protein called Rep A binds to a section of the double-stranded DNA called the origin of replication or Ori.
Rep A is encoded by a plasmid gene.
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| Rolling circle model of replication |
Rep A nicks one strand of the DNA and holds on to the 5' end of the strand.
The 3' end with its free (OH) hydroxyl group serves as a primer for a host DNA polymerase to begin to replicate the intact complementary strand.
The rep A initiator protein recruits a helicase that unwinds the DNA. As the DNA unwinds it becomes coded by single strand DNA binding proteins.
As replication proceeds the nicked strand which continues to be covered with single strand DNA binding proteins, progressively peels off until replication of the intact strand is complete.
The two ends of the nicked single strand are rejoined by the Rep A protein and released.
DNA ligase seals the nick in the double-stranded molecule.
The single-stranded DNA can now be replicated. A region of the DNA becomes looped allowing RNA polymerase access to the DNA to form a primer.
Host DNA polymerases use the primer as a starting point for the synthesis of DNA.
Finally DNA ligase seals the remaining nick resulting in a double-stranded plasmid.
Each of these plasmids can undergo replication again by the same method.
