In the early 1940s, American researcher Oswald Avery and his colleagues set out to test whether the transforming agent in Griffith's experiment was protein, RNA, or DNA. The scientists used enzymes to separately destroy each of the three molecules in heat-killed S cells. They used a protease enzyme to destroy protein in heat-killed cells in the first experiment, an enzyme called RNase to destroy RNA in the second experiment, and an enzyme called DNase to destroy DNA in the third experiment. Then, they separately mixed the three experimental batches of heat-killed S cells with live R cells and injected mice with the mixtures.

Avery and his group found that the cells missing protein and RNA were able to transform R cells into S cells and kill the mice. However, cells missing DNA did not transform R cells into S cells, and therefore the mice survived. They concluded that DNA is responsible for transformation in bacteria.

35S-labeled protein

Experiment 1



35S radioactivity did not enter bacterial cell.


Protein is not the hereditary material.

Bacterial cell

32P-labeled DNA

Experiment 2

Label bacteriophages with radioactive isotopes. Allow phages to infect bacterial cells.

32P radioactivity entered bacterial cell.

Use blender to remove viruses' coat from surface of bacterial cell.

32P radioactivity entered bacterial cell.

Centrifuge to separate heavier bacteria from lighter phages.

DNA is the hereditary material.

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