Final answer:
Frederick Griffith's experiment with Streptococcus pneumoniae revealed that non-pathogenic R strain cells transformed into pathogenic S strain after exposure to heat-killed S strain, due to the transfer of genetic material, a process called transformation.
Step-by-step explanation:
In the 1920s, Frederick Griffith conducted an experiment with two strains of Streptococcus pneumoniae: the non-pathogenic R strain and the pathogenic S strain, which has a capsule that contributes to its virulence. He injected mice with a combination of heat-killed S strain cells, which should not cause disease, and live R strain cells. Unexpectedly, the mice died, and live S strain cells were recovered from their bodies.
Griffith hypothesized that the R strain must have taken up something from the heat-killed S strain that transformed them into the virulent S strain. This process was termed transformation, indicating that external DNA from the S strain was incorporated into the R strain cells, altering their morphology and virulence. Griffith's findings were foundational in understanding genetic material's role in transferring traits between organisms.