The rolling circle model describes DNA replication in circular DNA. It involves three steps: initiation, elongation, and termination. In initiation, the circular DNA is nicked by helicase and topoisomerase. In elongation, the leading strand is elongated continuously while the lagging strand is displaced and replicated in fragments. In termination, the linear DNA molecule is cleaved from the circular DNA, resulting in a double stranded circular DNA molecule and a single stranded linear DNA molecule that is then circularized by ligase.

1. PRESENTED BY:
SYEDA TAMANNA YASMIN
DU2017MSC0130
M.Sc Microbiology,2nd sem

2. INTRODUCTION
 The rolling circle model for DNA replication suggests
that DNA must de circular.
 It describes a process of unidirectional nucleic acid
replication.
Three basic steps: (i) Initiation
(ii) elongation
(iii) termination

3. STEPS
 Circular dsDNA will be “nicked”.
 3’ end is elongated- leading strand
 5’ end is displaced- Lagging strand
 Replication of both “unnicked” and displaced ssDNA.
 Displaced DNA circulates and synthesis its own
complementary strand.

5. INITIATION
Phosphate ends, by action of
(a) Helicase
(b) Topoisomerase
(c) SSB proteins

6. ELONGATION
 -OH group of broken strand.
 The polymerase will start to move in a circle for
elongation
 End will be displaced
 It will grow like a waving thread.

7. TERMINATION
 The linear DNA molecule is cleaved from the
circular resulting in a :
(a) double stranded circular DNA molecule
(b) single stranded linear DNA molecule.
 the linear single stranded molecule is circularized
by the action of ligase and then replication to
double stranded circular plasmid molecule.

8. EXAMPLE: conjugation of F+ and F- bacteria

9. SYNTHESIS OF (+) STRAND
 Loop rolling replication.
 Begins with primosome added to gene A.
 Helicase attaches to (-) strand at gene A.
 Pol III extends the (+) strand from 3’-OH.
 Extension process generates a loop rolling structure.
 Old (+) strand is pelled off.
 Simultaneously newly formed 3’OH of old looped
strand attacks its 5’ phosphate terminus attaches to
5’protein A.

11. SYNTHESIS OF (-) STRAND
 The (+) strand is coated with SSB ,except for PAS.
 This recognized by pri A,pri B,pri C .
 Dna B and Dna C add to the DNA.
 Dna C then releases.
 Primose pro-pelled in 5’-3’direction along with (+)strand
by pri A.
 Pol III extends the primer.
 Pol I excises the primer.
 Fragments are joined by ligase and super coiled by
gyrase.
 Forms (-) strand.