DNA
(Deoxyribonucleic Acid)
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Genetic material of cells…
• GENES – units of genetic material that
CODES FOR A SPECIFIC TRAIT
• Called NUCLEIC ACIDS
• DNA is made up of repeating molecules
called NUCLEOTIDES
A HISTORY OF DNA
Discovery of the DNA double helix
• Frederick Griffith – Discovers that
a factor in diseased bacteria can
transform harmless bacteria into
deadly bacteria (1928)
• Rosalind Franklin – X-ray photo of
DNA (1952)
• Watson and Crick – described
the DNA molecules from Franklin’s
X-ray (1953)
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.
Chromosomes and DNA
• Our genes are on
our
chromosomes.
• Chromosomes
are made up of a
chemical called
DNA.
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.
The Double Helix Molecule
• The DNA double
helix has two
strands twisted
together.
• (In the rest of this
unit we will look at
the structure of one
strand.)
Nucleotides
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
One Strand of DNA
• One strand of DNA
is a polymer of
nucleotides.
• One strand of DNA
has many millions
of nucleotides.
nucleotide
One Strand of DNA
• The backbone
of the molecule
is alternating
phosphate and
deoxyribose.
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
Nitrogenous Bases
• PURINES
1. Adenine (A)
2. Guanine (G)
A or G
• PYRIMIDINES
3. Thymine (T)
4. Cytosine (C)
T or C
Two Kinds of Bases in DNA
• Pyrimidines are
single ring bases.
• Purines are double
ring bases
Thymine and Cytosine are
pyrimidines
• Thymine and cytosine each have one ring
of carbon and nitrogen atoms
Adenine and Guanine are
purines
• Adenine and guanine each have two rings
of carbon and nitrogen atoms.
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?
N
N
C
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.
C
N
Hydrogen Bonds
N
C
N
C
C
C
N
O
Important:
• Adenine and Thymine always
join together
A
T
• Cytosine and Guanine always
join together
C
G
Watson & Crick proposed…
•DNA had specific pairing between the
nitrogen bases:
ADENINE – THYMINE
CYTOSINE - GUANINE
•DNA was made of 2 long stands of
nucleotides arranged in a specific
way called the “Complementary Rule”
DNA Double Helix
5
O
3
3
O
P
5
O
C
G
1
P
5
3
2
4
4
2
3
1
P
T
5
A
P
3
O
O
P
5
O
3
animation
5
P
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
The earth is 150 billion m
sun more than 400
or 93 million miles from
times.
the sun.
• DNA has a diameter of
only 0.000000002 m.
Genetic Diversity…
• Different
arrangements of
NUCLEOTIDES in a
nucleic acid (DNA)
provides the key to
DIVERSITY among
living organisms.
The Code of Life…
• The “code” of the chromosome is the
SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…