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Search for Genetic Material

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Search for Genetic Material - Lesson Summary

Frederick Griffith, a British scientist, conducted an experiment that accidentally demonstrated the transformation of genetic material in bacteria. In fact, Frederick Griffith was in the process of developing a vaccine against the bacteria -Streptococcus pneumoniae that caused pneumonia, by inoculating the dead microbes along similar lines of Louis Pasteur’s experiments on vaccination. Frederick grew the bacteria in a culture plate and found that they were of two forms, the smooth and the rough. This was because the smooth shiny colonies or the S strain of the bacteria possessed a mucous or a polysaccharide coat, while the rough colonies or the R strain had no such coat. Further, when the mice were injected with the S strain, they died of pneumonia infection while those injected with the R strain did not develop pneumonia. This proved that the S strain of bacteria was virulent or lethal, while the R strain was harmless. Moreover, Frederick also found that heating killed the bacteria. So, when he injected heat-killed S strain bacteria into the mice, they did not die. On the other hand, when he injected a mixture of R strain bacteria and heat-killed S strain bacteria, the mice died. Surprisingly, he even recovered the living S strain of bacteria from the dead mice. So, on the basis of this experiment, Frederick concluded that the heat-killed S strain bacteria had somehow transformed the R strain bacteria. He also concluded that the reason why the R strain bacteria produced a smooth polysaccharide coat and became lethal or virulent was because of the transfer of some genetic material from the heat-killed S strain bacteria. However, there was still no means to ascertain or identify the genetic material that was transferred and responsible for the transformation process. Later, after sixteen years, in nineteen forty four, a team of three scientists Oswald Avery, Colin MacLeod and Maclyn McCarty, once again began research on the experiment that was once conducted by Frederick. Through their experiments, they discovered that it was the DNA from the S strain bacteria that had transformed the R strain bacteria. They also found that digestion of the DNA of heat killed S strain with DNase inhibited transformation. Further, the scientists also discovered that addition of proteases or protein-digesting enzymes and RNases or RNA-digesting enzymes did not affect the transformation. This led to the conclusion that proteins or RNA were not transforming substances and implied that DNA was the cause for the transformation. Thus, through their experiments, Oswald Avery, Colin MacLeod and Maclyn McCarty concluded that DNA is the hereditary material in most living organisms.


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