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Griffith and Avery, McCloud & McCarthy provide
the first evidence for DNA as the genetic material
Oswald Avery
Much of the first half of
the century microbiologists
attempted to identify
organisms responsible for
What is Pneumococcus?
The Pneumococcus is a bacteria that causes serious infections in adults
and children, including pneumonia, blood infections, and meningitis.
This bacteria is also the number one cause of sinusitis and ear
Fred Griffith
 wanted to make a vaccine against
Streptococcus pneumoniae, which caused a type
of pneumonia.Though he failed in making the
vaccine he stumbled on a demonstration of the
transmission of genetic instructions by a
process we now call transformation.
He found that the bacterium had two forms when grown
on agar plates, a smooth (S) and a rough (R) form.
The R bacteria were harmless, but the S bacteria were
lethal when injected into mice.
Heat-killed S cells were also harmless-the same effect
seen by Pasteur.
However, surprisingly when live R cells were mixed with killed
S cells and injected into mice the mice died, and the
bacteria rescued from the mice had been "transformed" into
S type.
This experiment strongly implied that genetic material
had been transferred from the dead to the live cell. It
was hard to be certain of this, or to know what the
material was in this crude experiment.
Sixteen years later the team of Avery, MacLeod
and McCarty revisited this experiment and
attempted a more definitive experiment.
A favorite hypothesis of the 1930s was that the
genetic instructions in chromosomes was protein.
However,chemical analysis of DNA had begun to
suggest that it might be a more interesting molecule
than had been supposed.
Avery, MacLeod and McCarty repeated Griffith's
experiment using purified DNA molecules and got
the same "transformation" of R cells into S cells.
Avery,Macleod and McCarty ‘s
An explanation of Griffith's results came in the 1940's with
the work of Oswald Avery, Colin MacLeod and Maclyn
McCarty. These researchers suggested that something in the
heat treated S bacteria transformed the R type bacteria into
living S. In order to demonstrate what the transforming
agent was, they took live R and heat treated S and mixed it
with two enzymes:
protease which destroys protein--notice from
the image that the mouse still died, indicating
that something other than protein was the
transforming agent.
DNase which destroys DNA--notice from the
image that the mouse lived, indicating that DNA
is required for the transformation event.
In short, these researchers demonstrated that
DNA, and not protein, was the transforming
In fact, it is unlikely that either Griffith or Avery
had any real feeling for how important their work would
ultimately become. Instead, each focused only on
answering a very specific question. Griffith wondered,
"Can bacteria change from one type to another?" and
showed that they could. Avery asked, "What substance
makes them change?" and found that it was DNA. Yet,
by asking simple questions and designing elegant
experiments to answer them, Griffith and Avery laid the
groundwork for a scientific revolution that neither could
have imagined.
Source :
The work by :
Chau Shuk Tak (3)
Tse Man Wai (26)