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1
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
2
History of
DNA
3
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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History of DNA
• Chromosomes are made
of both DNA and
protein
• Experiments on
bacteriophage viruses
by Hershey & Chase
proved that DNA was
the cell’s genetic
material
Radioactive
32P
was injected into bacteria!
5
Discovery of DNA
Structure
• Erwin Chargraff showed the
amounts of the four bases on DNA
( A,T,C,G)
• In a body or somatic cell:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
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Chargaff’s Rule
• Adenine must pair with
Thymine A - T
• Guanine must pair with
Cytosine G - C
• The bases form weak
hydrogen bonds
T
A
G
C
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DNA Structure
• Rosalind Franklin
took diffraction xray photographs of
DNA crystals
• In the 1950’s,
Watson & Crick built
the first model of
DNA using Franklin’s
x-rays
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Watson & Crick’s Model
• 1953 Watson &
Crick proposed
that DNA is
composed of two
strands of
nucleotides held
together by
nitrogenous bases
in the form of a
double helix.
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Why do we study DNA?
We study DNA for
many reasons,
e.g.,
• its central
importance to all
life on Earth,
• medical benefits
such as cures for
diseases,
• better food crops.
10
Chromosomes and DNA
• Our genes are on
our chromosomes.
• Chromosomes are
made up of a
chemical called
DNA.
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The Shape of the Molecule
• DNA is a very long
polymer.
• The basic shape is
like a twisted ladder
or zipper.
• This is called a
double helix.
12
The Double Helix Molecule
• The DNA double
helix has two
strands twisted
together.
• These strands
run antiparallel.
3’-5’ & 5’-3’
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One Strand of DNA
• The backbone of
the molecule is
alternating
phosphates and
deoxyribose
sugar
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
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Nucleotides
O
O -P O
O
O
O -P O
O
One deoxyribose together
with its phosphate and base
make a nucleotide.
O
O -P O
O
Phosphate
Nitrogenous
base
O
C
C
C
O Deoxyribose
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One Strand of DNA
nucleotide
• One strand of
DNA is a polymer
of nucleotides.
• One strand of
DNA has many
millions of
nucleotides.
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Four nitrogenous bases
DNA has four different bases:
C
• Thymine T
• Adenine A
• Guanine G
• Cytosine
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Two Kinds of Bases in DNA
• Pyrimidines are
single ring bases.
• Purines are
double ring
bases.
N
N C
O C
C
N C
N
N C
C
C
N
N C
N C
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Thymine and Cytosine are
pyrimidines
• Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
N
C
N
C
cytosine
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Adenine and Guanine are
purines
• Adenine and guanine each have two
rings of carbon and nitrogen atoms.
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
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Two Stranded DNA
• Remember, DNA
has two strands
that fit
together
something like a
zipper.
• The teeth are
the nitrogenous
bases but why
do they stick
together?
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C
N
N
C
N
C
C
C
C
N
N
N
C
C
C
O
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are weak
but there are millions
and millions of them in a
single molecule of DNA.
• The bonds between
cytosine and guanine are
shown here with dotted
lines
N
Hydrogen Bonds
N
O
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Hydrogen Bonds, cont.
• When making
hydrogen bonds,
cytosine always
pairs up with
guanine
• Adenine always
pairs up with
thymine
• Adenine is bonded
to thymine here
N
O
C
C
O
C C
N
C
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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.
The earth is 150 billion m
• DNA has a diameter of or 93 million miles from
the sun.
only 0.000000002 m.
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DNA and
Replication
25
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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Replication Facts
• DNA has to be copied before a cell
divides
• DNA is copied during interphase of
mitosis
• New cells will need identical DNA
strands
• Occurs in many spot on a
chromosome
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Three Phases of Replication
• Initiation: Unzipping DNA
• Elongation: Making new DNA strand
• Termination: Ends at telomeres
(specific repeated bases) Zipping
DNA back together.
DNA Replication
• Begins at Origins of Replication
• Two strands open forming Replication Forks (Yshaped region) The “unzipping of the DNA
strand is carried out by an enzyme DNA
helicase.
3’
• New strands grow at the forks
5’ Parental DNA Molecule
3’
Replication
Fork
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5’
DNA Replication
• Enzyme Helicase unwinds and
separates the 2 DNA strands by
breaking the weak hydrogen bonds
• Single-Strand Binding Proteins
attach and keep the 2 DNA
strands separated and untwisted
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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.
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
Direction of Replication
RNA
Primer
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5’
Leading and Lagging Strands
• Leading strand: starts at the 3’ end
of the parent DNA forming the
complementary strand of 5’ to 3’
• Lagging strand: is discontinuous
(patchwork for DNA); requires many
RNA primers for Okazaki fragments
Okazki Fragment
• The joining
of RNA
primers and
DNA
polymerase
creates an
Okazaki
fragment.
Only occurs
on lagging
strand
Finishing the New DNA
Strands
• DNA Ligase “zips up”
the lagging DNA
strand
– This is done by joining
the old and new
strands of DNA with
hydrogen bonds.
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Remember the Strands are
Antiparallel
O
5
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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Replication of Strands
Replication
Fork
Point of Origin
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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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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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DNA Damage & Repair
• Chemicals & ultraviolet radiation
damage the DNA in our body cells
– Specific or non specific repair
• 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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