Much of the first half of the century microbiologists attempted to identify organisms responsible for disease.

Griffith and Avery, McCloud & McCarthy provide the first evidence for DNA as the genetic material

Oswald Avery

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 infections.

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.

The R bacteria were harmless, but the S bacteria were lethal when injected into mice.

He found that the bacterium had two forms when grown on agar plates, a smooth (S) and a rough (R) form.

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 experiment

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 agent.

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 :http://mywebpages.comcast.net/biologycentury/pages/dna1.html

http://www.accessexcellence.org/

Chau Shuk Tak (3) Tse Man Wai (26)

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