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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