Introduction to DNA
(Deoxyribonucleic acid)
Questions at the beginning of
the 20th Century
• How do genes work?
• What are they made of, and how do
they determine the characteristics of
organisms?
• Are genes single molecules, or are they
longer structures made up of many
molecules?
• How did we figure out the answers to
these questions?
The discovery of the gene and
DNA:
Griffith Transformation
Experiment
• Like many experiments in science,
the discovery of the molecular nature
of the gene was a complete accident.
• In 1928, British scientist Fredrick
Griffith was trying to figure out how
bacteria made people sick.
Griffith Transformation
Experiment
• Griffith isolated two slightly
different strains of pneumonia
bacteria from mice.
• One strain that caused pneumonia
and one strain that did not cause
pneumonia.
• The pneumonia causing strain
grew on nutrient agar plates in
smooth colonies.
• The non-pneumonia causing
bacteria grew on nutrient agar
plates as colonies with rough
edges.
• This is how Griffith distinguished
the two types of bacteria.
Griffith Transformation
Experiment
• When Griffith injected
mice with the diseasecausing strain (smooth)
of bacteria, the mice
developed pneumonia
and died.
• When mice were injected
with the harmless strain
(rough), the mice didn’t
get sick.
• Griffith wondered if the
disease-causing bacteria
might produce poison.
Griffith Transformation
Experiment
• To find out, Griffith
took a culture of these
cells, heated the
bacteria to kill them,
and injected the heatkilled bacteria into the
mice.
• The mice survived!
• This suggested that the
cause of pneumonia
was not a chemical
poison released by the
bacteria.
Griffith Transformation
Experiment
• Next, Griffith mixed
heat-killed, diseasecausing bacteria with
live harmless bacteria
and injected the mixture
into mice.
• By themselves, neither
should have made the
mice sick
• To Griffith’s
amazement, the mice
developed pneumonia
and died.
Griffith Transformation
Experiment
• When Griffith examined
the lungs of the mice, he
found that the lungs
were infected with the
pneumonia-causing
strain.
• Somehow the heat
killed strain of bacteria
had passed on their
disease causing ability
to the harmless strain.
Griffith Transformation
Experiment
• Griffith called this
process
transformation
because one strain of
bacteria (the
harmless strain) had
apparently been
changed permanently
into another (the
disease-causing
strain).
Griffith Transformation
Experiment
• Griffith hypothesized
that when the living,
harmless bacteria
and the heat-killed
bacteria were
mixed; some factor
was transferred
from the heat-killed
cells into the live
cells.
Griffith Transformation
Experiment
• Those factors must
contain information that
could change harmless
bacteria into diseasecausing bacteria.
• Since the ability to cause
disease was inherited by
the transformed
bacteria’s offspring, the
transformation factor
might be a gene.
Transformation of Bacteria
AVERY and DNA
• In 1944, a group of scientists
led by Oswald Avery tried to
repeat Griffith’s work.
• They did so to determine
which molecule in the heatkilled bacteria was most
important in transformation.
• If transformation required
just one particular molecule,
that might be the molecule of
the gene.
• Avery and his colleagues
extracted all the “juice”
from the inside of the
heat-killed bacteria.
• They treated the extract
with enzymes that
destroy proteins, lipids,
carbohydrates, and
other molecules
including the nucleic acid
that makes up RNA.
• Transformation still
occurred.
AVERY and
DNA
AVERY and DNA
• Since all of the
molecules listed
had been destroyed,
they could not be
responsible for
transformation.
AVERY and DNA
• Avery and other
scientists performed
another experiment,
this time using an
enzyme that
destroyed the nucleic
acid that makes up
DNA.
• This time,
transformation did
not occur.
AVERY and DNA
• This lead to Avery coming
to the conclusion that
DNA was transforming
factor.
• Avery and other scientist
discovered that the
nucleic acid DNA stores
and transmits the
genetic information from
one generation of an
organism to the next.
Hershey-Chase Experiment
• In 1952, Alfred
Hershey and Martha
Chase studied
viruses to further
convince the
scientist that genetic
information is
passed along from
generation to
generation by DNA.
Hershey-Chase Experiment
• They collaborated in
studying viruses,
nonliving particles
smaller than a cell
that can infect living
organisms.
Hershey-Chase Experiment
• Bacteriophages: (bacteria
eater) viruses that infect
bacteria.
– Composed of DNA or RNA
core and a protein coat.
– When a bacteriophage enters
a bacterium, the virus
attaches to the surface of the
cell and injects its genetic
information inside the
bacteria.
Hershey-Chase Experiment
• The viral genes are
inserted into the host
DNA and tell the bacteria
cell to produce hundreds
of viruses.
• Gradually the viruses
destroy the bacterium
causing the cell to split
open releasing the
viruses.
Hershey-Chase Experiment
• Hershey and Chase
reasoned that if they
could determine which
part of the virus, the
protein coat or the DNA
core, entered the
infected cell, they
would learn whether
genes were made of
protein or DNA.
Hershey-Chase Experiment
• To do this, they grew viruses in
cultures containing radioactive
isotopes.
– Phosphorus – 32
– Sulfur - 35
• Protein contains very little
phosphorus and DNA contains
no sulfur.
Hershey-Chase Experiment
• These radioactive substances
could be used as markers.
– If sulfur – 35 was found in the
bacteria, the virus would have
injected its protein coat into
the bacterium.
– If phosphorus – 32 was found
in the bacterium, the virus
would have injected its DNA
into the bacterium.
Hershey-Chase Experiment
• The two scientists mixed
the marked viruses with
the bacterium and then
waited a few minutes so
the viruses had time to
inject their genetic
material.
• They separated the
viruses from the bacteria
and tested the bacteria for
radioactivity.
Hershey-Chase Experiment
• Nearly all of the
radioactivity found in
the bacteria was
from phosphorus –
32.
• Hershey and Chase
concluded that the
genetic material of
the bacteriophage
was DNA, not
protein.
Classwork
• Get a Biology Text
• Turn to page 294
• Answer questions 1 and 5