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DNA
DNA was identified as the genetic material
through a series of experiments
Molecular Genetics

Study of the chemistry of the materials of
inheritance.
Structure of DNA
 Contains information to construct the
molecules needed for life (proteins).

Griffith and Transformation

1928 Frederick Griffith was researching
pneumonia.
Griffith finds a ‘transforming principle.’
Griffith experimented with the bacteria that
causes pneumonia.
 Two types:
1. S = deadly (Sick)
2. R = harmless
 TRANSFORMATION occurred
 S changed R into harmful

Griffith’s Experiment
Heat killed disease
causing bacteria
Disease
causing
bacteria
Harmless
Bacteria
Heat Killed
bacteria
No
growth
Harmless bacteria
MOUSE
CROAKS!
Transformation
Griffith mixes heat-killed disease causing
bacteria with live harmless bacteria
 The mice died
 A factor from the heat killed bacteria had
“transformed” *(CHANGED) the
harmless bacteria into disease causing
ones.

Griffith’s Experiment

Experiment Video
Avery and DNA
Oswald Avery – 1944
 Repeats Griffith’s experiment
 Wants to know what caused the
transformation?
 Uses enzymes to break down or
destroy parts of the pneumonia
causing bacteria

Avery and DNA
Parts of Bacteria
Proteins
RNA
DNA
Did transformation
occur afterwards?
Conclusion
Avery's Experiment

Enzyme to break down proteins


Enzyme to break down RNA


Transformation occurred-
Transformation occurred
Enzyme to break down DNA

NO TRANSFORMATION OCCURRED
Avery and DNA

Conclusion:
 DNA caused transformation!
 If is causes transformation then:
 Could DNA be genetic material?
The Hershey-Chase Experiment

Alfred Hershey and
Martha Chase -1952
 Studied viruses


non-living particles
smaller than a cell
can infect living
organisms
1952 Alfred Hershey and
Martha Chase
Is genetic material made of DNA or
Protein?
 Used radioactive sulfur 35 and
phosphorus 32
 Bacteriophage

The Hershey-Chase Experiment

Bacteriophage – a
virus that infects and
kills bacteria
(bacteria eater)

Composed of:
DNA or RNA
core
 protein coat

The Hershey-Chase Experiment
Bacteriophage with
phosphorus-32 in DNA
Bacteriophage with sulfur-35
in protein coat
Phage infects
bacterium
Phage infects
bacterium
Radioactivity
inside bacterium
No radioactivity
inside bacterium
The Hershey-Chase Experiment

Video
The Hershey-Chase Experiment

Conclusion: The genetic material is DNA
Francis Crick and James Watson
1953

Developed a 3-D
model of DNA
 Used information
from other scientists
 Two strands are
wound around each
other
 Looks like:
 Twisted ladder
 Spiral staircase
Erwin Chargaff’s Rules 1950
Erwin Chargaff – American Biochemist
 In any sample of DNA



Percentages of cytosine and guanine are almost equal
Percentages of thymine and adenine are almost equal
 C=G and T=A
Percentages of Bases in Four Organisms
Sources of DNA
Streptococcus
Yeast
Herring
Human
A
29.8
31.3
27.8
30.9
T
31.6
32.9
27.5
29.4
G
20.5
18.7
22.2
19.9
C
18.0
17.1
22.6
19.9
X-Ray Evidence
Early 1950’s
 Rosalind Franklin
used x-ray on DNA
 X-shaped pattern
suggests - two
strands in the
structure

Francis Crick and James Watson
1953

DNA is shaped as a
double helix.
 Two strands are
wound around each
other
 Looks like:
 Twisted ladder
 Spiral staircase