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Avery, MacLeod and McCarthy experiment

Griffith could not understand the cause of bacterial transformation.

Avery, MacLeod and McCarthy experiment:

Griffith could not understand the cause of bacterial transformation. Oswal T Avery, C.M.MacLeod and M.J.McCarthy identified the transforming principle in 1944. Avery and his coworkers standardized a procedure to extract active power from liquid cultures of type III S virulent cells. From the original 75 liter sample, the procedure yielded 10 to 25 mg of the active factors. The “active factor” that was a fibrous mass was analyzed for its Nitrogen/Phosphorus ratio and was formed to coincide with the ratio of DNA.

The final extracted product was also treated with the proteolytic enzymes trypsin and chymotrypsin and then with an RNA digesting enzyme, ribonuclease. Such treatments destroyed the activity of pro-teins and RNA, and the transforming activity still remained.

The final confirmation was done by treating the crude samples with DNA digesting enzyme deoxyribonuclease. Digestion with this enzyme destroyed the transforming activity. This proved that the trans-forming principle was DNA.


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