Introduction to Bacterial DNA
Bacterial cells carry a large chromosomal DNA molecule and often contain smaller circular DNA molecules called plasmids. Plasmids replicate autonomously, meaning their replication does not depend on the timing of chromosomal replication. This independence allows plasmids to maintain themselves in the cell even when the host chromosome is not actively dividing.
The Rolling Circle Replication Process
Initiation and Nicking
The replication initiator protein RepA binds to the plasmid’s origin of replication (ori). RepA is encoded by a gene located on the plasmid itself. After binding, RepA nicks one strand of the double‑stranded DNA and remains attached to the 5′ end of the nicked strand. The 3′ end, now bearing a free hydroxy group, serves as a primer for the host DNA polymerase.
Strand Displacement and Synthesis
RepA recruits a helicase that unwinds the DNA ahead of the replication fork. As the DNA unwinds, single‑strand DNA binding proteins coat the exposed single strand, protecting it from degradation. Host DNA polymerase uses the 3′ hydroxy primer to synthesize a new complementary strand while the nicked strand is progressively displaced, creating a growing single‑stranded tail.
Completion and Circularization
When the intact complementary strand is fully synthesized, RepA rejoins the two ends of the displaced single strand. DNA ligase then seals the nick in the original double‑stranded molecule, producing a circular plasmid with a looped single‑stranded tail. RNA polymerase binds the looped strand and creates an RNA primer. Host DNA polymerase extends from this primer to synthesize the second strand, and DNA ligase seals the final nick, yielding a complete double‑stranded plasmid ready for segregation.
Detailed Sequence of Rolling Circle Replication
- Binding: RepA protein binds to the origin of replication (ori).
- Nicking: RepA nicks one strand; the 3′ hydroxy group serves as a primer.
- Unwinding: Helicase unwinds DNA; single‑strand binding proteins coat the displaced strand.
- Synthesis: Host DNA polymerase replicates the intact complementary strand.
- Release: RepA rejoins the ends of the displaced strand; DNA ligase seals the original molecule.
- Second Strand Synthesis: RNA polymerase forms a primer on the displaced single strand; DNA polymerase synthesizes the complementary strand; DNA ligase seals the final nick.
Takeaways
- Plasmids are autonomous circular DNA molecules that replicate independently of the bacterial chromosome.
- RepA binds the plasmid origin, nicks one strand, and holds the 5′ end while the 3′ hydroxy group primes host DNA polymerase.
- Helicase unwinds the DNA, single‑strand binding proteins coat the displaced strand, and host polymerase synthesizes the complementary strand as the nicked strand is peeled off.
- RepA rejoins the two ends of the displaced strand and DNA ligase seals the original molecule, creating a looped single‑stranded DNA.
- RNA polymerase primes the looped strand, a second DNA polymerase synthesizes the complementary strand, and DNA ligase seals the final nick, yielding a complete double‑stranded plasmid.
Frequently Asked Questions
What role does RepA play in rolling circle replication?
RepA initiates the process by binding the plasmid’s origin of replication, nicking one DNA strand, and remaining attached to the 5′ end. This creates a free 3′ hydroxy group that serves as a primer for host DNA polymerase, and later RepA rejoins the displaced strand’s ends to release the newly synthesized DNA.
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