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| Experiment to prove that DNA is Genetic Material Griffith Effect |
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| The fact that DNA is genetic material came from the experiments using bacteria and viruses. |
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| The first series of experiments were performed by a British bacteriologist F. Griffith in 1928, using the bacterium Diplococcus pneumoniae which causes pneumonia in mammals. |
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| Griffith noticed that this bacterium had two types of strains. |
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 S-type, which was capsulated and produced a smooth colony on a synthetic medium. |
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 R-type, which was non-capsulated and produced rough colony on a synthetic medium. |
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| When S-type of bacteria was injected into healthy mice, the mice developed pneumonia and died. So S-type was named as virulent or pathogenic. |
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| However, R-type of bacteria was non-pathogenic. |
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| If heat killed S-type of bacteria were injected into healthy mice, they did not cause disease and the mice remained healthy. |
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| When heat killed S-type of bacteria were mixed with R-type living bacteria and the mixture injected into healthy mice, the mice developed pneumonia and died. |
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| When bacteria were isolated from the dead mice, they were of living S-type and R-type. |
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| Griffith announced that it was because of a phenomenon other than mutation, which he called transformation. |
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| In 1944, O. T. Avery, McCleod and McCarty repeated the experiments of Griffith and found that when living R cells were mixed with the capsule of heat killed S type and infected into mice, there was no disease. |
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| But when they injected a mixture of R cells and the chromosome of S-bacteria into mice, the mice developed pneumonia and died. |
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| This led to the conclusion that the chromosome of S-bacteria causes the transformation and not the capsule. |
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| So they announced that bacterial transformation involves transfer of a part of DNA from the dead bacterium (donor) to the active living bacterium (recipient), which expresses the character of the donor cell, and so is called a recombinant. |
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| Summary of the Experiment |
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| Experiment of Avery and others showing that DNA is the Transforming Agent |
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