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DNA Structure
 DNA consists of two molecules that are arranged
into a ladder-like structure called a Double
Helix.
 In 1950’s, James Watson & Francis Crick
determined model of DNA
 A molecule of DNA is made up of millions of
tiny subunits called Nucleotides.
 Each nucleotide consists of:
1. Phosphate group
2. 5-carbon (Pentose) sugar
3. Nitrogenous base
Nucleotides
Phosphate
Nitrogenous
Base
Sugar
DNA Nucleotide
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
C2
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Nitrogenous base
(A, G, C, or T)
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Nucleotides
 The phosphate and sugar form the backbone
of the DNA molecule, whereas the bases form
the “rungs”.
 There are four types of nitrogenous bases.
Nucleotides
A
Adenine
C
Cytosine
T
Thymine
G
Guanine
Nucleotides
 Each base will only bond with one other
specific base.
 Adenine (A)
 Thymine (T)
 Cytosine (C)
 Guanine (G)
Form a base pair.
Form a base pair.
DNA Structure
 Because of this complementary base pairing,
the order of the bases in one strand
determines the order of the bases in the other
strand.
Chargaff’s Rule
 Erwin Chargaff – Scientist: Discovered base-
pairing rules
 The bases form the “rungs” on the DNA ladder by
complementary pairing
A == T
C == G
T == A
G == C
A == T
T == A
Sugarphosphate
backbone
A
T
C
G
T
A
C
G
A
T
G
C
T
A
Nitrogenous Bases
 Double ring PURINES
Adenine (A)
Guanine (G)
A or G
 Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
T or C
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Four Nitrogen Bases of DNA
 Pyrimidines:
Nitrogeneous bases
That have a SINGLE
Ring of Carbon and
Nitrogen Atoms.
Ex. Cytosine and
Thyamine
 Purines:
Nitrogeneous bases
That have a Double
Ring of Carbon and
Nitrogen Atoms.
Ex. Adenine and
Guanine
Base-Pairings
 Purines only pair with Pyrimidines
 Three hydrogen bonds required to bond
Guanine & Cytosine
3 H-bonds
G
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C
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DNA Structure
 To crack the genetic code found in DNA we
need to look at the sequence of bases.
 The bases are arranged in triplets called
codons.
AGG-CTC-AAG-TCC-TAG
TCC-GAG-TTC-AGG-ATC
DNA Structure
 A gene is a section of DNA that codes for a
protein.
 Each unique gene has a unique sequence of
bases.
 This unique sequence of bases will code for the
production of a unique protein.
 It is these proteins and combination of proteins
that give us a unique phenotype.
DNA
Gene
Protein
Trait