DNA and
Replication
1
Rosalind Franklin
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Chargaff’s Rule
• Adenine must pair with
Thymine
• Guanine must pair with
Cytosine
• The bases form weak
hydrogen bonds
T
A
G
C
3
DNA
• Two strands coiled called
a double helix
• Sides made of a pentose
sugar Deoxyribose bonded
to phosphate (PO4) groups
• Center made of nitrogen
bases bonded together by
weak hydrogen bonds
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Three Parts of the DNA
Nucleotide
Phosphate
Group
O
O=P-O
O
CH2
O
N
C
C
Sugar
(deoxyribose)
C
C
Nitrogenous base
(A, G, C, or T)
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DNA
O
O
P
P
O
O
P
C
G
O
T
P
A
O
P
P6
Question
• If there is 30% Adenine,
how much Cytosine is
present?
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Answer
• There would be 20%
Cytosine
• Adenine (30%) = Thymine
(30%)
• Guanine (20%) = Cytosine
(20%)
• Therefore, 60% A-T and
40% C-G
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DNA
Replication
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Replication Facts
• DNA is copied in a cell before
a cell divides by mitosis,
meiosis, or binary fission.
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DNA Replication
• Begins at Origins of Replication
• Two strands open forming Replication
Forks (Y-shaped region)
• New strands grow at the forks
5’ Parental DNA Molecule
3’
3’
Replication
Fork
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5’
DNA as a Zipper
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DNA Replication
• Enzyme Helicase unwinds
and separates the 2 DNA
strands by breaking the
weak hydrogen bonds
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Question
• How does the structure of DNA
make it easy to copy?
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DNA Replication
• DNA polymerase (enzyme) adds
the new nucleotides
• Covalent bonds form between
deoxyribose sugar and phosphate
group.
• Hydrogen bonds between
nitrogenous bases.
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Zipper Movement
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Question
• Are the nucleotides of the new
sequences of the new strand
identical to the original?
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Synthesis of the New DNA
Strands
• The Leading Strand is
synthesized as a single strand
from the point of origin toward
the opening replication fork
5’
3’
Nucleotides
DNA Polymerase
5’
RNA
Primer
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Synthesis of the New DNA
Strands
• The Lagging Strand is made in MANY
short segments It is replicated from
the replication fork toward the origin
• Leaves gaps in new strand. Gaps filled
in by DNA ligase
Leading Strand
5
’
3’
DNA Polymerase
5’
3’
Lagging Strand
RNA Primer
3’
5’
3’
5’
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Replication of Strands
Replication
Fork
Point of Origin
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Semiconservative Model of
Replication
• New DNA consists of 1
PARENTAL (template) and 1 NEW
(complementary) strand of DNA
DNA Template
Parental DNA
New DNA
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Question
• Why is the new strand
complementary to the original
strand?
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Question
• After a cell’s DNA is replicated,
the cell may divide in two. Each
new cell receives one copy of
the original cell’s DNA. How are
the new strands and the original
strands divided between the two
new cells?
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• Explain the statement,
“DNA replication depends
on specific base pairs”.
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Activity:
• Is photography a document
similar to DNA replication?
Think of the original materials,
the copying process, and the
final products. Explain how the
two processes are alike.
Identify major differences.
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