The document discusses the structure and composition of DNA. It explains that DNA contains the blueprint of life in the form of instructions for making proteins. DNA has a double helix shape with two strands coiled around each other. Each strand consists of repeating nucleotide units containing a phosphate, deoxyribose sugar, and one of four nitrogenous bases: adenine, guanine, cytosine, or thymine. The bases on each strand bond with each other through hydrogen bonds, with adenine bonding only to thymine and cytosine bonding only to guanine. This ensures the fidelity of DNA replication and transmission of genetic information from parent cells to daughter cells.

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DNADNA
• DNADNA is often
called the
blueprint of life..
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.

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Watson & Crick’s Model

4. 4
Why do we study DNADNA?
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.

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Chromosomes and DNAChromosomes 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 MoleculeThe 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.

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The Double Helix MoleculeThe Double Helix Molecule
• The DNA
double helix
has two
strands
twisted
together.

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One Strand of DNAOne 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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NucleotidesNucleotides
C C
C
O
Phosphate
O
C
C
O -P O
O
O
O -P O
O
O
O -P O
O
O
One deoxyribose together
with its phosphate and base
make a nucleotide.
Nitrogenous
base
Deoxyribose

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One Strand of DNAOne Strand of DNA
• One strand of
DNA is a polymer
of nucleotides.
• One strand of
DNA has many
millions of
nucleotides.
nucleotide

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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 areare
single ring basessingle ring bases..
• PurinesPurines areare
double ringdouble ring
bases.bases.
C
C
C
C
N
N
OO
N
C
C
C
C
N
N
N
N
N
C

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Thymine and Cytosine areThymine and Cytosine are
pyrimidinespyrimidines
• Thymine and cytosine each have oneThymine and cytosine each have one
ring of carbon and nitrogen atoms.ring of carbon and nitrogen atoms.
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 areAdenine and Guanine are
purinespurines
• AdenineAdenine andand guanineguanine each have twoeach have two
rings of carbon and nitrogen atoms.rings of carbon and nitrogen atoms.
C
C
C
C
N
N
N
AdenineAdenine N
N
C
C
C
C
C
N
N
OO
N
GuanineGuanine N
N
C

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Two Stranded DNATwo 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
C
C
C
N
N
OO
N
C
C
C
C
N
N
OO
N
N
N
C
Hydrogen BondsHydrogen Bonds
• 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

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Hydrogen Bonds,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
C
C
C
C
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 and Thymine
always join together
A T
• Cytosine and Guanine
always join together
C 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 m
or 93 million miles from
the sun.

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