Download DNA Replication

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DNA by the Numbers
 Each cell has about 2 m
of DNA.
 The average human has
75 trillion cells.
 The average human has
enough DNA to go from
the earth to the sun
more than 400 times.
 DNA has a diameter of
only 0.000000002 m.
The earth is 150 billion m
or 93 million miles from
the sun.
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DNA – deoxyribonucleic acid is
the nucleic acid that stores and transmits
genetic info. from one generation to the
next.
•present in all organisms, but different
(unique) in each individual, except for
identical twins.
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DNA
 DNA is often called
the blueprint of
life.
 In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
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History of DNA
 Early scientists thought protein was
the cell’s hereditary material because
it was more complex than DNA
 Proteins were composed of 20
different amino acids in long
polypeptide chains
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Transformation
 Griffith 1928-worked with virulent S
and nonvirulent R strain Pneumoccocus
bacteria
 found that R strain could become
virulent (transform) when it took in
DNA from heat-killed S strain
 Avery 1944- suggested that DNA was
probably the genetic material that was
“transformed”
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Griffith Experiment
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History of DNA
 Chromosomes are made of
both DNA and protein
 Hershey & Chase 1952experiments on
bacteriophage viruses
proved that DNA was the
cell’s genetic material
Radioactive
32P
was injected into bacteria!
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Chargaff’s Rule
 Adenine pairs with Thymine
 Guanine pairs with Cytosine
 The bases form weak hydrogen
bonds
T
A
G
C
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Discovery of DNA
Structure
 Erwin Chargaff 1950 showed the
amounts of the four bases on DNA
(A,T,C,G)
 In a body or somatic cell: If…
A = 15% then…
T = __% so…
G = 35%
C = __%
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DNA Structure
 Rosalind Franklin & Maurice
Wilkins - 1952 took diffraction
x-ray photographs of DNA
crystals (2 sides, twisted)
 Watson & Crick -1953 built the
first model of DNA using
Franklin’s x-rays
(double helix)
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Rosalind Franklin
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Watson and Crick
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Helix
 Most DNA has a right-hand
twist with 10 base pairs in a
complete turn
 Left twisted DNA is called
Z-DNA or southpaw DNA
 Hot spots occur where right
and left twisted DNA meet
producing mutations
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Antiparallel Strands
 One strand of
DNA goes from
5’ to 3’ (sugars)
 The other
strand is
opposite in
direction going
3’ to 5’ (sugars)
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The Shape of the Molecule
 DNA is a very long
double stranded
polymer of
nucleotides.
 The basic shape is
like a twisted
ladder or zipper.
 This is called a
double helix.
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One Strand of DNA
 Each nucleotide monomer P
nucleotide
B
contains
B
P
1 phosphate group
1 sugar (deoxyribose)
B
*1 nitrogenous base
P
 One strand of DNA has many
millions of nucleotides.
P
B
P
B
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DNA Nucleotide
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
C2
Nitrogenous base
(A, G, C, or T)
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DNA
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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One Strand of DNA
 The backbone
of the molecule is
alternating
phosphates &
deoxyribose sugar
(phosphodiester bond)
 The “rungs/teeth” are
nitrogenous
bases (4 possible)
C, T, A, or G
phosphate
deoxyribose
bases
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4 possible Nitrogenous Bases
 Double ring PURINES
Adenine (A)
Guanine (G)
A or G
 Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
T or C
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Base-Pairings
 Purines only pair with Pyrimidines
 Hydrogen bonds required to bond
Guanine & Cytosine
3 H-bonds
G
C
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•Hydrogen bonds are required
to bond Adenine & Thymine
2 H - bonds
T
A
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DNA Double Helix
“Rungs of ladder”
Nitrogenous
Base (A,T,G or C)
“Legs of ladder”
Phosphate &
Sugar Backbone
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Two Stranded DNA
 DNA has two
strands that fit
together
something like a
ladder or zipper.
 The rungs or
teeth are the
nitrogenous
bases but why do
they stick
together?
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DNA
 Two strands coiled called a
double helix
 Sides made of a pentose sugar
Deoxyribose bonded to
phosphate (PO4) groups
 Middle made of nitrogen bases
bonded together by weak
hydrogen bonds
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Hydrogen Bonds
 The bases attract each
other because of hydrogen
bonds.
 Hydrogen bonds are
weak but there are
millions of them in a
single molecule of
DNA. Cytosine always
pairs with Guanine
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Hydrogen Bonds, cont.
 When making
hydrogen bonds,
Cytosine
always pairs
with Guanine

Adenine
always pairs
with Thymine
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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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The nitrogen bases
of each strand pair
with the bases on the
complementary
strand
The order of the bases
makes up the genetic
code.
A
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Question:
 What would be the
complementary DNA strand for
the following DNA sequence?
DNA –C G T A T G-
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Answer:
DNA –C G T A T Gcomp DNA –G C A T A C-
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Question:
If there is 30% Adenine,
how much Cytosine is
present?
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Answer:
If 30% Adenine then
30% Thymine
 If 60% A-T; then 40% C-G
 Therefore,40% C-G would be
20% Guanine = __% Cytosine
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DNA
REPLICATION
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When a cell divides,
DNA preserves individuality
by passing exact copies of
itself to the new cell
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Replication Facts
 DNA has to be copied
before a cell divides
 DNA is copied during the S
or synthesis phase of
interphase
 New cells will need identical
DNA strands
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Synthesis Phase (S phase)
 S phase during interphase of the
cell cycle
 Nucleus of eukaryotes
DNA replication takes
place in the S phase.
S
phase
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
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.
5’ to 3’ Sugars
When the DNA double helix
unwinds, it resembles a ladder
The sides of the ladder are
the sugar-phosphate
backbones
The rungs of the ladder are
the complementary paired
bases
The two DNA strands are
anti-parallel (they run in
opposite directions)
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Steps in DNA Replication
Occurs when chromosomes duplicate
(make copies)
Enzyme DNA Helicase unwinds & separates
the 2 DNA strands by breaking the weak
hydrogen bonds as enzymes “unzip” the
molecule
Each old strand of nucleotides serves
as a template for each new strand
New nucleotides move into
complementary positions are joined by
DNA polymerase
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AntiParallel
Strands
of DNA
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DNA Replication
 Begins at Origins of Replication
 One strand serves as a mold for
another strand to be copied
 Two strands open forming Replication
Forks (Y-shaped region)
 New strands grow at the forks
5’ Parental DNA Molecule
3’
3’
Replication
Fork
5’
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Two New,
Identical
DNA Strands
Result from
Replication
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Another View of
Replication
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DNA Replication
 Begins at Origins of Replication
 One strand serves as a mold for
another strand to be copied
 Two strands open forming Replication
Forks (Y-shaped region)
 New strands grow at the forks
5’ Parental DNA Molecule
3’
3’
Replication
Fork
5’
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DNA Replication
 1.Enzyme DNA Helicase
unwinds & separates the 2 DNA
strands by breaking the weak
hydrogen bonds to unzip the chain
 Single-Strand Binding Proteins attach and keep the 2
DNA strands separated and untwisted
2. Free nucleotides match up &
form H bonds to complete
complementary base strand
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3. Base pairs bond, DNA polymerase
links phosphate of one nucleotide to
the sugar of another.
4. Pairing continues until each
“original” DNA strand has a
complete matching strand
(result 2 identical DNA strands)
IF…
TAGCAT
ATCGTA
THEN…
TAGCAT
ATCGTA
and
TAGCAT
ATCGTA
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Replication of Strands
Replication
Fork
Point of Origin
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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 strand of DNA
DNA Template
Parental DNA
New DNA
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DNA Replication



Before new DNA strands can form,
there must be RNA primers present to
start the addition of new nucleotides
Primase is the enzyme that
synthesizes the RNA Primer
DNA polymerase can then add
the new nucleotides
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DNA Replication
 DNA polymerase can only add nucleotides
to the 3’ end of the DNA
 This causes the NEW strand to be built in
a 5’ to 3’ direction
5’
3’
Nucleotide
DNA Polymerase
RNA
Primer
5’
Direction of Replication
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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
RNA
Primer
5’
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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’
DNA Polymerase
5’
3’
Lagging Strand
RNA Primer
3’
5’
3’
5’
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Remember HOW the Carbons
Are Numbered!
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
Nitrogenous base
(A, G, C, or T)
C2
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Lagging Strand Segments
 Okazaki Fragments - series of short
segments on the lagging strand
 Must be joined together by an enzyme
DNA
Polymerase
Okazaki Fragment
RNA
Primer
5’
3’
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
Lagging Strand
Okazaki Fragment 2
3’
5’
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Proofreading New DNA
 DNA polymerase initially makes about 1 in
10,000 base pairing errors
 Enzymes (helicase) 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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DNA Damage & Repair
 Chemicals & ultraviolet radiation
damage the DNA in our body cells
 Cells must continuously repair
DAMAGED DNA
 Excision repair occurs when any of
over 50 repair enzymes remove
damaged parts of DNA
 DNA polymerase and DNA ligase
replace and bond the new nucleotides
together
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DNA makes proteins that are needed
for growth, repair and all life functions
Ex:
collagen - cartilage and tendons
hemoglobin – blood carries oxygen
through the body
keratin - hair and fingernails
insulin – metabolizes blood sugars
…muscles, skin, etc…
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DNA  RNA  Protein
DNA
Transcription
mRNA
Ribosome
Translation
Protein
Prokaryotic Cell
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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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DNA
 Stands for
Deoxyribonucleic acid
 Made up of subunits called
nucleotides
 Nucleotide made of:
1. Phosphate group
2. 5-carbon sugar
3. Nitrogenous base
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Question:
 What would be the
complementary DNA
strand for the following
DNA sequence?
DNA 5’-CGTATG-3’
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Answer:
DNA 5’-CGTATG-3’
DNA 3’-GCATAC-5’
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