Aim #16: How does a DNA
molecule replicate itself?
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1.
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2. Replication Facts
• DNA has to be copied
before a cell divides
• New cells will need identical
DNA strands
• DNA serves only as a
template.
• Enzymes and other proteins
do the actual work of
replication.
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• DNA REPLICATION ANIMATION
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3. Starting Replication
a) Begins at Origins of Replication
b) Two strands open forming Replication
Forks (Y-shaped region)
3’
5’ Parental DNA Molecule
3’
Replication
Fork
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5’
3. Starting Replication
c. Enzyme DNA Helicase
“unzips” and separates the
2 DNA strands by
breaking hydrogen bonds
d.Single-Strand Binding
Proteins attach and keep
the 2 DNA strands
separated and untwisted
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Each unzipped strand is a template, or
pattern, for a new molecule of DNA to form
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4. RNA Primase
• RNA primers: before new DNA
strands can form, there must be RNA
primers present to start the addition
of new nucleotides (by DNA
Polymerase).
• RNA Primase: enzyme that
synthesizes the RNA primer.
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5. DNA Polymerase
• DNA Polymerase: with a RNA primer in
place, DNA Polymerase (enzyme) adds
new nucleotides to the DNA template in the
5’ to 3’ direction
– DNA polymerase also “proofreads” as it adds
nucleotides to make sure it is copied correctly
5’
3’
Nucleotide
DNA Polymerase
Direction of Replication
RNA
Primer
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5’
6. Leading and Lagging
Strands
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Remember the Strands are
Antiparallel
5
O
3
3
P
5
O
O
C
G
1
P
5
3
2
4
4
P
5
P
2
3
1
O
T
A
3
O
3
5
O
5
P
P
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Leading and Lagging Strands
• The Leading Strand is
synthesized as a single strand
from the point of origin in a 5’
to 3’ direction
5’
3’
Nucleotides
DNA Polymerase
5’
RNA
Primer
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Leading and Lagging Strands
• Lagging Strand: also synthesized in
the 5’ to 3’ direction, but discontinuously
against overall direction of replication
• This strand is made in MANY short segments
Leading Strand
5
’
3’
DNA Polymerase
5’
3’
Lagging Strand
RNA Primer
3’
5’
3’
5’
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7. Okazaki Fragments
• Okazaki Fragments - series of
short segments on the lagging
strand
• Must be joined together by an
enzyme
DNA
Okazaki Fragment
RNA
Primer
5’
3’
Polymerase
Lagging Strand
3’
5’
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Joining of Okazaki Fragments
• The enzyme Ligase joins the
Okazaki fragments together to
make one strand
DNA ligase
5’
3’
Okazaki Fragment 1
Okazaki Fragment 2
3’
5’
Lagging Strand
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Leading and Lagging
Strands
Replication
Fork
Point of Origin
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8. Ligase
• Joins Okazaki Fragments
• Replaces RNA primer with DNA
nucleotides
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9. Proofreading New
DNA
• DNA polymerase initially makes
about 1 in 10,000 base pairing
errors
• Enzymes proofread and correct
these mistakes
• The new error rate for DNA that
has been proofread is 1 in 1 billion
base pairing errors
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10. Semiconservative Model
of Replication
• Idea presented by Watson & Crick
• The two strands of the parental
molecule separate, and each acts as a
template for a new complementary
strand
• New DNA consists of 1
PARENTAL (original) and 1 NEW
DNA Template
strand of DNA
Parental DNA
New DNA
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11. Prokaryote vs Eukaryote
Begins at one pointcontinues in two
directions until
chromosome is
completely copied
Begins at multiple pointscontinues in both directions
until chromosome is
completely copied
(replication bubbles)
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