Rolling circle model Introduction Summary of DNA rolling circle

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