DNA Replication Semiconservative
6.2 DNA Replication
DNA carry coded
information in its
sequence of bases
DNA could be
precisely copied, a
process known as
DNA replication
LEARNING OUTCOMES :
At the end of this topic, students should be
able to:
(a) describe semi-conservative replication of
DNA.
(b) describe DNA replication during cell
division.
1
Overview of DNA replication
Each strands of DNA
molecule could serve
as a template, for the
synthesis of the new
daughter strands
2
DNA replication
Replication begins at special sites called
origins of replication
In eukaryotic cell, the process is speeded up
by having multiple origin of replication
3
Unwinding DNA
• DNA helix strands separate by enzymes
–DNA helicase and DNA gyrase
• Each separated strands is bound by
- single-stranded binding proteins
DNA helicase
& DNA gyrase
4
Copying the DNA
• Each DNA / both
parental strand
serves a template
• There are many
points of origins
• The replication
bubbles grow in both
directions until two
daughter DNA
molecules are formed
5
single-stranded binding proteins
replication fork
6
1
Replication bubble grows in two directions
5′′
3′′
3′′
5′′
• Protein complex primase add in RNA
primer from scratch
• DNA polymerase III add in new DNA
nucleotide (one at a time) specific
base pairing complementary to the
template strand
DNA polymerase III
5′′
3′′
5′′
5′′
3′′
Leading strand
Overall directions
of replication
Helicase
5′′
Lagging strand
Lagging strand
Single-strand
binding protein
leading strand
3′′
Overview
Origin of replication
Leading strand
Leading strand
5′′
3′′
lagging strand
5′′
3′′
DNA pol III
3′′
Primer
3′′
3′′
5′′
lagging strand
5′′
5′′
Parental DNA
Primase
3′′
DNA pol III
growing
replication fork 5′′
3′′
growing
replication fork
5′′
5′′
leading strand
4
3′′
leading strand
5′′
lagging strand
5′′ 5′′
Lagging strand
5′′
3′′
3′′
DNA pol I
DNA ligase
5′′
5′′
3
2
1
5′′
3′′
7
•
Adding new DNA nucleotides
The lagging strand is
synthesized discontinuously
away from the replication fork
forming Okazaki fragments
• (Note that free
nucleotides are added
to the free 3’-OH end
of the short RNA
primer)
Lagging strand
Leading strand
Lagging strand
Single-strand
binding protein
• Elongation is in the
direction of 5’
Overview
Origin of replication
Leading strand
Overall directions
of replication
Helicase
Leading strand
3’
5′′
• The leading strand is
3′′
DNA pol III
3′′
Primer
synthesized
continuously
towards the
replication fork
5′′
Parental DNA
Primase
3′′
DNA pol III
Lagging strand
5′′
4
3′′
DNA pol I
DNA ligase
5′′
5′′
3
2
1
3′′
9
Replacing RNA primers with DNA nucleotides
DNA polymerase I
removes sections of RNA primer and replaces with DNA nucleotides
The RNA primer is replaced with
DNA nucleotide by DNA
polymerase I
The Okazaki fragments are joined
by DNA ligase.
DNA ligase
Seals the discontinous fragments
5′′
5′′
5′′
ligase
ligase
Helicase
5′′
RNA
growing
3′′
replication fork
Leading strand
Overall directions
of replication
Leading strand
3′′
RNA
RNA
Lagging strand
Lagging strand
Single-strand
binding protein
DNA polymerase I
3′′
Overview
Origin of replication
Leading strand
3′′
DNA pol III
3′′
ligase
Primer
Parental DNA
5′′
Primase
3′′
DNA pol III
Lagging strand
5′′
RNA
5′′
4
3′′
DNA pol I
5′′
3
ligase
DNA ligase
5′′
2
1
3′′
3′′
11
2
DNA helicase and DNA gyrase
Overview
separate double helix DNA
Origin of replication
strands
Lagging strand
Leading strand
single stranded binding protein
(SSB) bind to separated DNA
strand to while replication occurs
DNA polymerase III starts DNA
strand
synthesis
at 3′′ end ofLeading
primer,
Lagging strand
continues Overall
in 5′′ directions
→ 3′′ direction
of replication
DNA polymerase I replaces
RNA primer with DNA nucleotide
Enzymes and protein involved
in DNA replication
• Resulting in two identical
copies of the original /
parental DNA strand.
Single-strand
binding protein
Helicase
Leading
strand
& gyrase
5′′
3′′
Primase synthesizes
a short RNA primer
DNA helicase joins the
(Okazaki) fragments
DNA pol III
3′′
Primer
Parental DNA
5′′
Primase
3′′
DNA pol III
Lagging strand
5′′
4
3′′
DNA pol I
5′′
3
13
DNA ligase
5′′
2
1
3′′
14
3