DNA replication is the process by which DNA copies itself. It occurs in three steps: initiation, elongation, and termination. During initiation, proteins help unwind and separate the DNA double helix at the origin of replication. In elongation, DNA polymerase adds nucleotides to each strand while using the other strand as a template, creating two new DNA molecules. For the lagging strand, RNA primers help initiate DNA synthesis in fragments called Okazaki fragments. Termination occurs when the replication fork meets with another one, stopping further DNA synthesis.

1. DNA REPLICATION
LOPAMUDRA MOHAPATRA
B.Sc(BIOTECH) 3rd yr
ROLL NO:11303
SUBMITTED TO:
TANMAYEE SAMANTARAY

2. CONTENTS:
 INTRODUCTION OF DNA:

chemical composition

structure of DNA
 BASIC RULE FOR REPLICATION OF ALL
NUCLEIC ACIDS
 DNA REPLICATION:

Initiation

Elongation

Termination

3. INTRODUCTION OF DNA:
DNA: deoxy ribose nucleic acid
DNA is a genetic material.
DNA is a double stranded
helical structure.
DNA is also called as a
heriditary material.

5.  CHEMICAL COMPOSITION:
 DNA isolated from variety of plants ,animals ,bacteria & virus has shown a
complex form of polymeric compounds containing four monomers known as
deoxy ribonucleotide monomers or deoxyribotids.
 Each deoxy ribonucleotide consists of a pentose sugar ,a phosphate group & a
nitrogenous base.
 NITROGENOUS BASE:
 There are 2 nitrogen bases.
i.
Purine
It is of two types.
1.Adenine(6-amino purine)
2.Guanine(2-amino-6-oxo purine)
i.
Pyrimidine
It is of 3 types.
3.Thymine(2,4-dioxo-5-methyl pyrimidine)
4.Cytosine(4-amino-2-oxo pyrimidine)
5.Uracil(2,4-dioxo pyrimidine)

7.  STRUCTURE OF DNA:
 In 1953 James Watson & Hary Crick worked on the
DNA.
 They gave the double helical structure of DNA by the 3
techniques.
1.X-ray diffraction technique.
2.Base pairing.
3.Analytical
 DNA consist of two helical structure of polynucleotide
& it is a right handed double helix.
 The two strands of double helix are antiparallel.

8.  Each polynucleotide chain has a sugar phosphate backbone with
nitrogenous bases directed inside the helix.
 The nitrogenous bases of two antiparallel polynucleotide
strands are linked through hydrogen bonds.
 There are 2 hydrogen bonds between A &T.
 There are 3 hydrogen bonds between G &C.
 The hydrogen bonds are not only the attractive forces between
the two polynucleotide of double helix but also serves to hold
the structure together.
 The two polynucleotides in a double helix are complementary.
 Ten base pairs occur per turn of helix .The spacing between
adjacent base pairs 10angstrom.The helix is 20angstrom in
diameter.

10. BASIC RULE FOR REPLICATION OF
ALL NUCLEIC ACID:
The prime role of any mode of
replication is to produce the base
sequence of the parent molecule.
The specificity of base pairing-adenine
with thymine & guanine with cytosineprovides the mechanism used by all
replication systems.

11. DNA REPLICATION:
DNA replication is a means to produce new
molecules that have the same base
sequence.
Occurs during interphase of the cell cycle.
DNA replication is semi-conservative
• The parent DNA strand separates into two
• Each strand serves as a template for new
complementary strands
• The new double helix is half original

12.  SEMICONSERVATIVE REPLICATION OF
DOUBLE SRANDED DNA:
The purpose of DNA replication is to create
daughter DNA molecules that are identical to
the parental molecule.
Originally,two modes of replication were
suggested which differ in whether or not the
two single strands of the parent molecule
become rearranged after one round of
replication .These were called the
semiconservative & conservative modes.

14.  It is hydrogen bonded to its parental template.
 In the semiconservative mode, first proposed by Watson &
Crick, each parental DNA strands serve as a template for
one new or daughter strand & each new strand is formed.
 This mechanism ensource that genetic information will be
copied faithfully at each cell division.
 This process includes 3 steps.
o Initiation
o Elongation
o Termination

15. The origin of replication is called as
Ori C. This has two conserved regions
13 nt & 9 nt.
13 nt region has 3 copies & 9 nt region
has 5 copies. These regions are also
called as Okazaki Fragment.

17. INITIATION:
DnaA is a protein which will bind to the Ori C at the 9 nt
regions. This binding occurs in every copy.
To this DnaA complex , a complex of protein named DnaB
,will bind. That DnaA complex along with DnaB protein
forms prepriming complex.
The DNA has a coiled structure. This is uncoiled by a helicase
named DnaB .While uncoling the knot moves forward&
gradually the DNA opens.
But when the coil won’t open further, at this stage a tersional
stress or turgidity acts ,which is reduced to topoisomerase .
Thus ,uncoiling contains & gradually , replication initiates.

18. ELONGATION:
In this whole process DNA polymerase III is the main
replicating enzymes .This enzymes has 5’->3’ & 3’->5’
exonuclease activity.
RNA polymerase enzymes synthesis a RNA primer .This
primer is attached to the lagging strand in the 5’->3’ site
.This primer has 4-15 nt.
After the primer gets attached, the DNA polymerase
enzymes gets attached on the free 3’ end.
The Okazaki Fragments formation takes place.
The complementary strand formation takes place in the
leading strand. The strand extends upto 1000 bp.

19. RNA primer
RNA primase
3’
5’

20.  After RNA primer is in place DNA nucleotides are
added by DNA polymerase III.
 Eventually, the segment of DNA will run into another
RNA primer.
 The DNA segments are called Okazaki fragments.
Okazaki fragment
RNA primer
DNA polymerase III
3’
5’

21.  Once Okazaki fragments are formed DNA polymerase
I replaces the RNA primer with DNA nucleotides.
 DNA ligase links the fragments together.
5’
3’
New strand
Parent strand
3’
5’

22.  The enzymes(polymerase III holoenzymes) i.e the main
replicating enzyme has 3 main subunits α,ε,θ.
 The accessory subunits are γ,δ, δ’,ψ,x,β.
present on template DNA
 α-primers are present on the core α.
ε-3’->5’ exonuclease activity.
θ-function is unclear.
β-helps in clamping action.
γ-frameshift action.
 γ-subunits helps to attach & dettach the enzyme from the
lagging strands.
 SSB(single stranded binding) protein gives stability &
protects SSDNA.
 This SSB gets removed automatically.

23. DNA Replication (the lagging strand)
 In the lagging strand DNA replication is discontinuous
 DNA is replicated in segments that become joined
together.

24. DNA replication (the leading strand)
 Replication is continuous
 There are no fragments
 Helicase unwinds and unzips the double helix
 DNA polymerase III adds nucleotides in the direction of
5’→3’
 DNA polymerase only works in the direction of 5’→3’

25. TERMINATION:
For termination , first a termination point is
determined . This is done by TUS protein.
TUS PROTEIN:
• If replication in one side is faster than the other ,
then TUS is used.
• There are many B-strands in the TUS protein ,
which doesn’t allow the further replication of the
strand.
• They are unlinked by the topoisomerase-IV.

27. REFERENCE:
MOLECULAR BIOLOGY
by
FREIFELDER