DNA replication occurs through a semi-conservative process where the parental double helix unwinds and each strand serves as a template to produce two new DNA molecules, each with one original and one new strand. Replication begins at multiple origins of replication and proceeds bidirectionally. Enzymes such as helicase unwind the DNA and single-strand binding proteins stabilize the separated strands. DNA polymerase adds complementary nucleotides to the 3' ends of the new strands which grow toward each other, producing daughter strands. The leading strand is continuous while the lagging strand is synthesized discontinuously in short Okazaki fragments later joined by DNA ligase.

1. DNA Respiration
Model
Alexis vite
KEY
Guanine
Thymine
Adenine
Cytosine

2. Origins of replication
Replication Bubbles:
Hundreds of replicating bubbles (eukaryotes)
(Prokaryotes) Single replication fork (bacteria)
Parental Strand
Origin of replication
Daughter strand
Bubble
In Eukaryotes, DNA
replication begins at
many sites along the
giant DNA molecule of
each chromosome.
Bubble
Replication Fork

3. Strand Seperation:
1.Helicase: enzyme which catalyze the unwinding and
separation (breaking H- Bonds ) of the parental double helix.
2. Single- Strand Binding Proteins: proteins which attach
and help keep the separated strands apart.
Unwind DNA
Helicase enzyme
unwinds part of DNA helix
stabilized by single-stranded binding proteins
3. Semi-conservative replication
1. Each new DNA molecule contains one old strand &
one old strand

4. 1. Complementary base
pairing
DNA
Guanine- a compound
that occurs in guano
and fish scales, and is
one of the four
constituent bases of
nucleic acids. A purine
derivative, it is paired
with cytosine in
double-strand DNA.
5’
3’
G-C
Thymine- a compound that
is one of the four
constituent bases of
nucleic acids. A pyrimidine
derivative, it is paired with
adenine in double-stranded
DNA.
T-A
C-G
A-T
T-A
Adenine- a compound
that is one of the four
constituent bases of
nucleic acide. A purine
derivative,it is paired
with thymine in
double-stranded DNA.
T-A
G-C
A-T
3’
Makes replication possible
C-G
A-T
5’
Cytosine- A compound
found in living tissue as a
constituent base of
nucleic acids. It is paired
with guanine in doublestranded DNA.

5. Base Pairing in DNA
Purines
Adenine(A)
Guanine(G)
Pryimidines
Thymine(T)
Cytosine (C)
Pairing
A:T
2 bonds
C:G
3 bonds
Two strands coiled called a double
helix
Sides made of a pentose sugar
Deoxyribose bonded to phosphate
(po4) groups by phosphodiester
bonds.
Center made of nitrogen bases
bonded together by weak
hydrogen bonds.

6. Enzyme
DNA
Enzyme Topoisomerase attaches to
the forks of the bubble to relieves
stress on the DNA Molecule by
allowing free rotation around a single
strand.

7. DNA
5’
3’
T-A
C-G
A-T
3’
5’
G-C
T-A
DNA Helicase
(unwinds DNA)
C-G
A-T
T-A
T-A
T-A
Parental DNA molecule
G-C
Origins of replication
Replication Forks: hundreds of
Y-shaped regions of replicating DNA
molecules where new strands are
growing.
1. Uncoil & unzip
DNA molecule
enzyme breaks weak
2. Hydrogen Bond
between bases
T -A
G-C
A-T
Starts in origin of
Replication
helicase
5’
3’
5’
Begins at Origins of Replication
Two strands open forming Replication Forks (y-shaped region)
New strands grow at the forks.
G-C
A-T
3’
Replication
Fork

8. DNA polymerase
Sugar-Phosphate
backbone
5’
3’
Reads 3’- 5’
synthesises 5’3’
G-C
Base Pair (Joined by
Hydrogen bonding)
Old Strand
T-A
C-G
Replication of DNA
*base pairing
allows each strand to
serve as a template for a
new strand.
T-A
T
*new strand is ½ parent
template & ½ new DNA.
Nucleotide about
to be added to a
new strand
A-T
A-T
Each parent strand
remains intact
G-C
5’
New Strand
3’
Every DNA molecule is
half “old” and half “new”

9. Synthesis of the new DNA Strands:
DNA Polymerase: with a RNA primer in place,
DNA Polyymerase (enzyme) catalyze the synthesis
of a new DNA strand in the 5’ to 3’ direction.
5’
3’
5’
Nucleotide
DNA Polymerase
RNA Primer

10. Leading Strand:
The leading Strand is synthesized as a single strand
from the point of origin toward the opening replication
fork.
5’
3’
5’
Nucleotide
DNA Polymerase
RNA Primer

11. Synthesis of the New DNA Strands
The Lagging Strand is synthesized discontinuously
against overall direction of replication
This strand is made in MANY short segments it is
replicated from the replication fork toward the origin.
Leading Strand
5’
3’
5’
DNA Polymerase
RNA Primer
3
’
5’
5’
3’
Lagging strand

12. Okazaki Fragments: series of shore segments on the
lagging Strand.
Must be joined together by an enzyme.
Okazaki Fragment
DNA Polymerase
RNA Primer
3
’
5’
5’
3’
Lagging Strand

13. DNA ligase: a linking enzyme that catalyzes the formation of a
covalent bond from the 3’ to 5’ end of joining strands
Example: joining two Okazaki fragments together.
DNA
ligase
Okazaki Fragment 1
Okazaki Fragment 2
3
’
5’
5’
3’
Lagging Strand

14. Double Helicase Strand
5’
3’
5’
G-C
3’
G-C
T-A
C-G
A-T
A-T
T-A
T-A
T-A
T-A
G-C
G-C
A-T
3’
T-A
C-G
Sugar
phosphate
backbone
A-T
5’
3’
5’
Base Pairs
Guanine
Thymine
Adenine
Cytosine