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DNA
• DNA is the
blueprint of life.
• DNA contains the
instructions for
making proteins
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Watson & Crick’s Model
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Why do we study DNA?
• all life on Earth
depends on DNA
to pass on genetic
traits and run
cells
• medical benefits
such as cures for
diseases,
• better food crops.
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Chromosomes and DNA
• genes are on
chromosomes.
• Chromosomes are
made of DNA.
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The Shape of the Molecule
• DNA is a very long
polymer.
• Basic shape is a
twisted ladder
• called a double helix.
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The Double Helix Molecule
• The DNA
double helix
has two
strands
twisted
together.
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One Strand of DNA
• The backbone of
DNA 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 plus
phosphate and base make a
nucleotide.
O
O -P O
O
Phosphate
Nitrogenous
base
O
C
C
C
O Deoxyribose
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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 and have one
ring only
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 and have two rings
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
• The 2 DNA fit
together like a
zipper.
• Why do the
bases stick
together?
• Hydrogen bonds!
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C
N
N
C
N
C
C
C
C
N
N
N
C
C
C
O
• Bases are attracted to
each other by hydrogen
bonds.
• Hydrogen bonds are weak
attractive forces
between a hydrogen on
one strand and a
nitrogen or oxygen on
the other strand of
DNA.
N
Hydrogen Bonds
N
O
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Hydrogen Bonds, cont.
• Cytosine pairs
with guanine using
3 H bonds
• Adenine pairs with
thymine using
• 2 H bonds
N
O
C
C
O
C C
N
C
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Chargraff’s Rule:
• Adenine with Thymine
A
T
• Cytosine with Guanine
C
G
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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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