Molecular Biology of the
Gene
DNA
 Identification


Structure of DNA
DNA Replication
of Genetic Material
Genetic Material –DNA or Protein?

Bacteriophage Replication
Martha Chase and Alfred Hershey (1952)
Roles of the Genetic Material
“A genetic material must carry out two jobs:
duplicate itself and control the
development of the rest of the cell in a
specific way.”
-Francis Crick
Hershey and Chase Experiment
Bacterial Transformation
Frederick Griffith, 1928

Diplococcus pneumoniae infects mice

Mice develop pneumonia and die
Two types of bacteria:
 R bacteria rough coat  no pneumonia
 S bacteria smooth coat  pneumonia
 Coat type is associated with virulence.
Griffith’s Experiments
Figure 9.1
The “Transforming Principle”
Avery, MacLeod, and McCarty, 1944
• Treated lysed S bacteria with protease and DNase
• DNase prevented transformation
• Therefore DNA is the transforming principle
Figure 9.2
Monomers and Polymers

Polymers are made up of monomers

Mono = one

Poly = many
For example:
Proteins are made up of amino acids

Polynucleotides

Polynucleotides are made up of nucleotides
Sugar-phosphate backbone
Phosphate group
A
A
C
C
Nitrogenous base
Sugar
DNA nucleotide
Nitrogenous base
(A, G, C, or T)
Phosphate
group
T
T
Thymine (T)
G
G
Sugar
(deoxyribose)
T
T
DNA nucleotide
DNA polynucleotide
Basic Structure of a Nucleotide
Phosphate Group
Nitrogenous Base
Sugar
Nitrogenous Bases (DNA)
Cytosine (C)
Thymine (T)
Adenine (A)
Guanine (G)
Pyrimidines
Purines
One Ring
Two Rings
And the Nobel Prize Goes To…

Physiology or Medicine 1962
"for their discoveries concerning the
molecular structure of nucleic acids
and its significance for information
transfer in living material"
Rosalind Franklin (1920-1958)
Watson and Crick with their model of DNA
Rope Ladder Model
Rope Ladder Model
Sugar and Phosphate
Backbone
Nitrogenous Base Pairs
Complementary Base Pairs
A-T
G-C
DNA: The Double Helix
C
G
T
A
T
A
Base
pair
C
Hydrogen bond
T
G
C
G
A
T
A
C
G
C
G
T
T
C
A
G
A
A
T
A
T
A
G
A
Ribbon model
T
C
T
Partial chemical structure
Computer model
Orientation of DNA
The 5’ phosphate of one
nucleotide is attached to
the 3’ hydroxyl group of
the previous nucleotide
The directionality of a
DNA strand is due to
the orientation of the
phosphate-sugar
backbone
Figure 9.11

Structure of DNA
 DNA

Replication
DNA vs. RNA
Chromosomes and Mitosis
Chromosomes Are Made of DNA
DNA Replication

DNA Replication is Semiconservative
A
T
A
T
C
G
C
G
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
A
C
C
Nucleotides
Both parental
strands serve
as templates
T
A
T
A
T
G
C
G
C
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
Two identical
daughter molecules
of DNA
Each new double helix
contains one parental
strand and one daughter
strand
G C
A T
G
C
C
G
A T
C
An enzyme “unzips” DNA
Replication Bubble
Replication Bubbles
Origin of replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
One Little Problem…
3 end
5 end
DNA is
synthesized
from 5’ to 3’
P
HO
5
4
3
2
1
2
A
T
5
P
C
P
P
G
C
P
P
T
3 end
3
4
G
P
OH
1
A
P
5 end
DNA Polymerase Can Only Move 5’ to 3’
DNA polymerase adds nucleotides to the 3’ end
“ase” = enzyme
DNA polymerase
molecule
3
5
LEADING STRAND
Daughter strand synthesized
continuously
Parental DNA
5
3
3
5
LAGGING STRAND
Daughter strand
synthesized In pieces
DNA Ligase
LEADING STRAND
5
3
5
3
LAGGING STRAND
DNA ligase
DNA ligase “glues” the fragments together


Structure of DNA
DNA Replication
 DNA
vs. RNA
Nitrogenous Bases (DNA and RNA)
H
Uracil (U)
Cytosine (C)
Thymine (T)
Adenine (A)
Guanine (G)
Pyrimidines
Purines
One Ring
Two Rings
DNA vs. RNA
Deoxyribonucleic Acid
Phosphate Group
Nitrogenous Base
Ribonucleic Acid
Phosphate Group
Nitrogenous Base
H
CH3
Thymine
Deoxyribose
H
Ribose
OH
Nitrogenous Bases
Nitrogenous Bases
T C A G
U C A G
Uracil