- Frederick Griffith’s 1928 experiments were a landmark study in biology, demonstrating the phenomenon of bacterial transformation.
- His work laid crucial groundwork for later discoveries that DNA is the genetic material, though Griffith didn’t know the transforming principle was DNA at the time.
- Griffith was studying different strains of Streptococcus pneumoniae to understand how it could cause disease (pneumonia).
- Griffith used two different strains of S. pneumoniae for his experiment:
- Smooth (S) strain: Virulent (disease-causing) strain, contain protective polysaccharide capsule
- Rough (R) strain: Non-virulent strain, lacks polysaccharide capsule
- Griffith injected mice with various combinations of bacterial strains. Mice injected with the S strain died, while those injected with either the live R strain or the heat-killed S strain survived. Surprisingly, when he injected a combination of the live R strain and the heat-killed S strain, the mice died.
- When Griffith examined the blood of the mice that died after being injected with the mixture, he found live S strain bacteria. This indicated that the non-virulent R strain had somehow transformed into the virulent S strain.
- Griffith concluded that something from the heat-killed S strain had transformed the live R strain into the virulent S strain. He called this the “transforming principle.” He hypothesized that this principle might be some part of the capsule or a substance required for capsule formation. However, He could not identify the exact nature of this transforming principle.
- The later discovery revealed that it was DNA that was transferred from the heat-killed S strain to the live R strain in Griffith’s experiment. This process of transformation allowed the non-virulent R strain to acquire the genetic information necessary to produce a protective capsule, thereby becoming virulent like the S strain. This discovery was pivotal in establishing DNA as the carrier of genetic information and laid the foundation for modern genetics.
