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The Structure of DNA
DNA



DNA (deoxyribonucleic acid) is found in
the nuclei of all cells.
It is the DNA that carries the genetic
information which will determine all
your characteristics (hair color, shape
of nose, etc.)
Every person’s DNA is different, except
for identical twins.
DNA is composed of:

5 carbon sugar, deoxyribose

phosphate group

4 different nitrogenous (nitrogencontaining) bases
P
S
Sugar and phosphate group
Nitrogenous Bases
Purines (double ring)
adenine
(A)
guanine
(G)
Pyrimidines (single ring)
thymine
(T)
cytosine
(C)
Purines
Pyrimidines
Each DNA unit is called a nucleotide.
A nucleotide consists of one phosphate
group, one deoxyribose molecule, and
one of the four bases.
Watson and Crick



James Watson and Francis Crick
discovered the structure of DNA in
1953.
It has been termed the discovery of the
century.
They determined its shape is a double
helix (like a spiral staircase)
Watson and Crick
The Double Helix


The sugars and phosphate groups are
bonded together to make up the
backbone of the DNA molecule. This is
like the handrails of the spiral staircase.
The bases bond to each other in pairs
and make up the internal structure of
the molecule. This is like the rungs or
steps of the spiral staircase.
DNA
Double
Helix

Adenine always bonds to Thymine.
A

T
Cytosine always bonds to Guanine.
C
G
DNA
Double
Helix


Bases are held together by weak
hydrogen bonds.
Adenine always bonds to thymine with
2 hydrogen bonds.
A

T
Cytosine always bonds to guanine with
3 hydrogen bonds.
C
G
DNA
Double
Helix

The two strands of the DNA molecule
are complementary – the sequence of
bases along one strand automatically
determines the matching bases on the
other strand because of the rules of
base pairings (A-T, C-G).
If all the DNA in your body were uncoiled
and stretched out, it would reach to the
sun and back ~3000 times!!!