DNA as Genetic Material: The Griffith Experiment
The concept of DNA as the genetic material was established through a series of key experiments, one of which was conducted by Frederick Griffith in 1928. Griffith’s experiment, known as the Griffith experiment, provided crucial evidence supporting the idea that DNA carries genetic information.
The Griffith Experiment
Griffith was studying Streptococcus pneumoniae, a bacterium that causes pneumonia in mice. This bacterium exists in two forms: a smooth (S) strain and a rough (R) strain. The smooth strain is virulent (disease-causing) due to the presence of a polysaccharide capsule that protects the bacteria from the immune system. The rough strain is non-virulent (does not cause disease) because it lacks this protective capsule.
Griffith conducted a series of experiments to understand how bacteria could transform from non-virulent to virulent forms. His experiment involved the following steps:
- Injecting Mice with S-strain Bacteria: Griffith first injected mice with live S-strain bacteria. As expected, the mice developed pneumonia and died, confirming the virulence of the S-strain.
- Injecting Mice with R-strain Bacteria: He then injected mice with live R-strain bacteria, which did not cause disease, and the mice survived.
- Injecting Mice with Heat-killed S-strain Bacteria: Next, Griffith injected mice with heat-killed S-strain bacteria. Since the bacteria were dead, the mice survived, showing that heat-killing destroyed the bacteria's ability to cause disease.
- Injecting Mice with Heat-killed S-strain and Live R-strain Bacteria: The most crucial part of the experiment was when Griffith injected mice with a mixture of heat-killed S-strain bacteria and live R-strain bacteria. Surprisingly, the mice died. Upon examining the dead mice, Griffith found live S-strain bacteria, indicating that the R-strain bacteria had somehow been "transformed" into virulent S-strain bacteria.
Conclusion: Transformation and Genetic Material
Griffith concluded that some "transforming principle" from the heat-killed S-strain bacteria had been transferred to the live R-strain bacteria, transforming them into the virulent S-strain. This experiment suggested that the genetic material responsible for inheritance was transferable and could change the properties of bacteria. Although Griffith did not know what the "transforming principle" was, his work laid the foundation for the later discovery that DNA is the genetic material, as further experiments by Avery, MacLeod, and McCarty would confirm.
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