Rolling circle replication is a form of DNA replication that occurs in circular DNA like plasmids, bacteriophages, and viroids. It involves five key steps: 1) a circular double-stranded DNA template is nicked at a single origin site, 2) the exposed 3' end is used to initiate replication of the leading strand while displacing the 5' end, 3) the displaced single-stranded DNA acts as the lagging strand and is replicated through Okazaki fragments, 4) both the displaced and unnicked strands are fully replicated, and 5) the displaced strand is rejoined to form a circular DNA product.

1. Rolling Circle Model
By- Dr. Ema Sushan
DNA Replication…..
(Molecular Biology)

2. Rolling Circle Replication
• Watson & Crick (1990s)
• Replication that is seen in circular DNA
• Only one origin
• Unidirectional
• Synthesize many copies of DNA and RNA
• Example :-
▫ Plasmids
▫ Bacteriophages
▫ Viroids

4. 1) Template is
circular duplex
DNA
4) Rejoining of nicked
strand
3) Elongation of
growing strand
displaces old
strand
RepA
Pol III
Pol III
3’OH
5’-P
5’-P
Growing Strand
Displaced Strand
2) Initiation occur
on one strand- nick
at origin
Displacement of nicked strand
DNA
Ligase
dsDNA
(synthesis of
first strand)
ssDNA
(synthesis of first
strand)

5. • Some eukaryotic viruses also replicate their DNA or RNA via
the rolling circle mechanism.
• Rolling circle DNA replication is initiated by an initiator protein
called RepA protein.
• The initiator protein remains bound to the 5' phosphate end of
the nicked strand, and the free 3' hydroxyl end is released to
serve as a primer for DNA synthesis by DNA polymerase III.
• Using the unnicked strand as a template, replication proceeds
around the circular DNA molecule, displacing the nicked strand
as single-stranded DNA.

6. • Helicase enzyme unwinds the strand and allows further
displacement of the nicked strand.
• SSB (single stranded DNA binding) proteins bind to the
displaced nicked strand.
• After displacement strand gets ligated by DNA ligase and
become circular.
• Later it form its complementary strand by forming loop and
removal of SSB protein takes place.

7. Summary of DNA rolling circle
Replication has five steps:
 Circular dsDNA will be "nicked".
 The 3' end is elongated using "unnicked" DNA as leading strand
(template); 5' end is displaced.
 Displaced DNA is a lagging strand and is made double stranded
via a series of Okazaki fragments.
 Replication of both "unnicked" and displaced ssDNA.
 Displaced DNA circularizes.

8. Thank you