Griffith's experiment
In 1928, Frederick Griffith conducted an experiment that showedtransformationliving cells bytransforming principle, which was later discoveredbe DNA.Griffith used two strainsPneumococcus (which infects mice),S (smooth) andR (rough) strain. The S strain covers itself withpolysaccharide capsule that protectsfromhost's immune system, resulting indeath ofhost, whileR strain doesn't have that protective capsuleis defeated byhost's immune system.
In his experiment, bacteria fromS strain were killed by heat,their remains were addedR strain bacteria. It turned out thatformerly harmless R strain now was ablekill its host. It had been transformed intolethal S strain, obviously bytransforming principle that was somehow part ofdead S strain bacteria.
Today, we know thatDNA ofS strain bacteria had survivedheating process,was taken up byR strain bacteria. The S strain DNA containsgenes that formprotective polysaccharide capsule. Equippedthis gene,former R strain bacteria were now protected fromhost's immune systemcould kill it.
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