oswald avery

Oswald Avery, a Canadian-born American bacteriologist,

discovered the material responsible for the "transformation principle." In 1944, at the Rockefeller Institute, he and his colleagues Maclyn McCarty and Colin MacLeod reported that the

"transforming principle" found by Fredrick Griffith, was DNA.

Avery first started his experiment by turning his attention to

Fredrick Grifilth. He worked with two strains of S. pneumonia.

One strand was surrounded by a polysaccharide capsule (virulent), and another didn't have a capsule (nonvirulent). The experiment showed that the virulent strand could convert the nonvirulent one into a carrier of the disease. This led to the discovery that the disease could be passed down to further generations, making it hereditary. This is when Oswald Avery stepped in to determine the type of substance that could make this transformation.

Many scientists initially believed that proteins

were the cause of transformation. However, an experiment conducted using the process of elimination proved otherwise. Identical extracted S-cells induced with heat were first given hydrolytic enzymes to destroy protein, RNA, and DNA. After this, they were mixed with live R cells. Surrounded S cells appeared in all the tests except the one treated with DNAse (an enzyme that breaks down DNA). The researchers then developed the hypothesis that DNA was the transforming principle.

Avery MacLeod McCarty Experiment

Heat-killed

S Cells

Lipids

Sugar

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Proteinase (No protein)

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RNase

(No RNA)

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DNase

(No DNA)

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R Cells

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R Cells

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R Cells

To confirm their hypothesis, they isolated DNA from a portion of the cell and noticed it possessed the same transforming characteristics as the previous heat-treated extract. This confirmed that DNA was the

"transforming principle" in cells.