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1
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DNADNA• 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 DNADNA?
• 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 DNAChromosomes and DNA
•genes are on chromosomes.
•Chromosomes are made of DNA.
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The Shape of the The Shape of the MoleculeMolecule
•DNA is a very long polymer.
•Basic shape is a twisted ladder
•called a double helix.
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The Double Helix The Double Helix MoleculeMolecule
•The DNA double helix has two strands twisted together.
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One Strand of DNAOne 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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NucleotidesNucleotides
C C
C
OPhosphate
O
CC
O -P OO
O
O -P OO
O
O -P OO
O One deoxyribose plus phosphate and base make
a nucleotide.
Nitrogenous base
Deoxyribose
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Four nitrogenous basesFour nitrogenous bases
• Cytosine C• Thymine T
• Adenine A
• Guanine G
DNA has four different bases:
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Two Kinds of Bases in DNA
• PyrimidinesPyrimidines are single ring are single ring basesbases..
• PurinesPurines are are double ring double ring bases.bases.
C
C
C
C
N
N
OO
N
CC
CC
N
N
N
N
N
C
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Thymine and Cytosine Thymine and Cytosine are pyrimidines and are pyrimidines and have one ring onlyhave one ring only
C
C
C
C
N
N
OO
N
cytosinecytosine
C
C
C
C
N
N
OO
OO
thyminethymine
C
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Adenine and Guanine Adenine and Guanine are purines and have are purines and have
two ringstwo rings
C
C
C
C
N
N
N
AdenineAdenine N
N
C
C
C
C
C
N
NOO
N
GuanineGuanine N
N
C
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Two Stranded DNATwo Stranded DNA• The 2 DNA fit
together like a zipper.
• Why do the bases stick together?
• Hydrogen bonds!
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C
C
C
C
N
N
OO
N
C
C
C
C
N NOO
N
N
N C
Hydrogen BondsHydrogen Bonds• 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.
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Hydrogen Bonds, Hydrogen Bonds, cont.• Cytosine pairs
with guanine using 3 H bonds
• Adenine pairs with thymine using
• 2 H bonds
C
C
CC
N
N
N
N
N
C
C
C
C
C
N
N
OO
OO
C
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Chargraff’s Rule:Chargraff’s Rule:
• Adenine with ThymineA T
• Cytosine with GuanineC G
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DNA by the NumbersDNA 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 mor 93 million miles from the sun.