Famous Early DNA Experiments…
Griffith…
In 1928, Fred Griffith performed the first experiment which suggested that protein was
not the genetic material. His experiment was actually fairly simple. He first injected mice
with a live strain of virulent (deadly) bacteria, and not to anyone's surprise, all of those
mice died. Then, he killed the virulent bacteria cells by heating them. Mice injected with
these heat-killed virulent bacteria did not die. In another set of mice, Griffith injected a
live non-virulent strain of bacteria, and these mice did not die, the result which Griffith
expected.
The surprise came when Griffith injected a group of mice with both live non-virulent
bacteria and heat-killed virulent bacteria. In that group, some of the mice died. When
Griffith examined those mice, he found live virulent bacteria in their blood. Griffith drew
the conclusion that the genetic information in the heat-killed virulent bacteria survived
the heating process and was somehow incorporated into the genetic material of the nonvirulent strain to cause them to become virulent. But Griffith knew that heat denatures
protein, so he suggested that the genetic material must be something else. However, his
results did not specifically point to DNA as a possibility.
Avery…
Oswald Avery followed up on Griffith's experiment in the following decade. Like
Griffith, Avery first used heat to kill virulent bacteria. He then extracted RNA
(ribonucleic acid), DNA, carbohydrates, lipids, and proteins from these dead cells, all of
which were considered to be possible candidates for the carriers of genetic information.
Next, he added each type of molecule to a culture of live non-virulent bacteria to
determine which was responsible for changing them into virulent bacteria as Griffith had
observed. Only the non-virulent cells which were given DNA from the dead virulent
strain became virulent, so Avery concluded that DNA must be the genetic material.
Hershey-Chase
The Hershey-Chase experiment was a series of experiments conducted in 1952 by Alfred
hershey and Martha chase that identified DNA to be the genetic material of phages and,
ultimately, of all organisms. A phage is a small virus that infects bacteria. It consists of a
protein coat that encloses the genetic material. When a phage infects a bacterium, it inserts
its genetic material into the bacterium, while its coat remains outside.
In a first experiment, T2 phages with radioactive
second experiment, T2 phages with radioactive
32P-labeled
35S-labeled
DNA infected bacteria. In a
protein infected bacteria. In both
experiments, bacteria were separated from the phage coats by blending followed by
centrifugation. In the first experiment, most radioactivity was found in the infected bacteria,
while in the second experiment most radioactivity was found in the phage coat. These
experiments demonstrated that DNA is the genetic material of phage and that protein does not
transmit genetic information.
Hershey shared the 1969 Nobel Prize in Physiology or Medicine for his discoveries concerning
the genetic structure of viruses
More detailed description…
Hershey and chase knew that T2 consisted of only DNA and protein, and that they somehow
manipulated host cells to produce new phages, but did not know if DNA or protein was
responsible.
Hershey and chase radioactively labeled E.Coli and T2 with
32P
(phosphorus isotope) and
35S
(sulfur isotope) because they knew DNA contains phosphorus, and protein contains sulfur, but
not vice versa.
They infected the
32P
E.Coli with
32P
T2 and
35S
E.Coli with
The results found that the progeny collected from the
isotope, while the
35S
32P
35S
T2, and collected the progeny.
E.Coli and
32P
T2 contained the
strains did not, providing more evidence that DNA was the genetic
information that bacteriophages inject into bacteria, not protein.
32P