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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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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.
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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.
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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
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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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 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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http://www.brainpop.com/he
alth/geneticsgrowthanddevelo
pment/dna/
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DNA
Replication
http://www.youtube.com/wat
ch?v=Kzgnl5-8WAk
copyright cmassengale
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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
copyright cmassengale
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Synthesis Phase (S phase)
• S phase during interphase of the
cell cycle
• Nucleus of eukaryotes
S
DNA replication takes
place in the S phase.
phase
G1
interphase
G2
Mitosis
copyright cmassengale
-prophase
-metaphase
-anaphase
-telophase
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• 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
copyright cmassengale
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Semiconservative Model of
Replication
• 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
copyright cmassengale
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• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=s
wf::535::535::/sites/dl/free/007243
7316/120076/micro04.swf::DNA%20
Replication%20Fork
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6B Genetic Code
• EVERY ORGANISM CONTAINS DNA
• The order(sequence) of the
nucleotides and the number of
chromosomes determines the type of
organism
• Ex: human has 23 chromosome pairs
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