Question
Describe Griffith's experiment for the demonstration of DNA as a genetic material with a properly labelled diagram.
Answer :
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Griffith’s experiment, conducted in 1928, was a landmark study in molecular biology that provided the first evidence for the existence of a “transforming principle,” later identified as DNA. He worked with *Streptococcus pneumoniae*, a bacterium responsible for pneumonia in mice, which existed in two strains: the smooth (S) strain with a polysaccharide capsule that caused disease, and the rough (R) strain without a capsule, which was non-virulent. The capsule in the S strain protected the bacteria from the host immune system, making it lethal to mice, while the R strain was harmless. Griffith injected four different preparations into separate groups of mice to observe their effects. In the first group, live S strain bacteria were injected, and the mice died, confirming the virulence of the S strain. In the second group, live R strain bacteria were injected, and the mice survived, showing the R strain was non-virulent. In the third group, heat-killed S strain bacteria were injected; the mice also survived, indicating that dead S bacteria alone could not cause disease. The crucial experiment was the fourth group, in which Griffith injected a mixture of live R strain bacteria and heat-killed S strain bacteria. Surprisingly, the mice died, and live S strain bacteria were recovered from their bodies. This indicated that some “transforming principle” from the dead S strain bacteria had converted the non-virulent R strain into virulent S strain bacteria. Griffith concluded that the genetic material from the dead S strain had transformed the R strain, transferring the virulence trait. This experiment was groundbreaking because it suggested that a chemical substance could carry genetic information and pass it between organisms. Although Griffith did not identify the chemical nature of the transforming principle, his work set the stage for later experiments by Avery, MacLeod, and McCarty, who demonstrated that DNA was responsible for transformation. The diagram representing Griffith’s experiment typically shows four groups of mice labeled according to the bacterial preparation injected:
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