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Question
Describe Avery, McLeod and McCarty’s experiment. State its significance.
Solution
In 1928, Frederick Griffith, an English medical bacteriologist, observed two strains of D. Pneumoniae. In one strain, the considerable amount of polysaccharide material is secreted by the cell which forms a large capsule around the cell. The colony produced by these cells has a glistening appearance and is called smooth (S) type. In the other strain, no polysaccharide slime layer is secreted by the cell. The colony formed by such cells has an irregular appearance and is said to be rough (R) type. The Strain is virulent and can cause pneumonia as the capsule protects the bacteria from mammalian immunological defence – phagocytosis by white blood cells. But the non-encapsulated R-strain is non-virulent as it can be inactivated by the defence mechanism.
Griffith observed that in the blood of dead mice both rough and smooth type of pneumococci occurs. He thus concluded that heat-killed smooth type bacteria caused a transformation of the living rough type bacteria. But he could not understand the cause of bacterial transformation.
Oswald T. Avery, C. M. MacLeod and M. J. McCarty repeated Griffith's experiments in an in vitro
system in order to identify the transforming substance responsible for converting non-virulent into virulent type. They found that the DNA isolated from heat-killed S-cells when added to R-cells changed their surface character from rough to smooth and also made them pathogenic. But when the extract was treated with DNA use (an enzyme which destroys DNA) the transforming ability. significance: DNA and not proteins the genetic material.
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