Here's a breakdown of Griffith's role:
Griffith's Experiment (1928):
1. Strains of Pneumococcus: Griffith worked with two strains of *Streptococcus pneumoniae*:
- Smooth (S) strain: Virulent, had a smooth capsule that protected it from the host's immune system.
- Rough (R) strain: Non-virulent, lacked a capsule, and was easily destroyed by the host's immune system.
2. Injecting Mice: He injected mice with different combinations of these strains:
- S strain: Mice died.
- R strain: Mice lived.
- Heat-killed S strain: Mice lived.
- Heat-killed S strain + R strain: Mice died!
3. Transforming Principle: This unexpected outcome showed that something from the heat-killed S strain had "transformed" the harmless R strain into a virulent form, allowing it to kill the mice. This "transforming principle" was later proven to be DNA.
Griffith's Significance:
* Evidence for Genetic Transfer: Griffith's experiment demonstrated that genetic information could be transferred from one bacterium to another, even after the donor bacterium was dead. This challenged the prevailing view that genetic information was solely passed down through inheritance.
* Foundation for DNA Research: Although he didn't identify the transforming principle itself, his work provided the crucial first step. Later scientists, notably Avery, MacLeod, and McCarty, built upon Griffith's work to identify DNA as the molecule responsible for this genetic transfer.
While Griffith's work didn't directly uncover the structure and function of DNA, it served as a critical turning point, paving the way for future research that ultimately led to the understanding of DNA's role as the fundamental unit of heredity.
