Final answer:
Fred Griffith discovered transformation in Streptococcus pneumoniae, a process where external DNA is absorbed by a cell, changing its characteristics. Using two strains, R (non-pathogenic) and S (pathogenic), he demonstrated that DNA from the killed S strain could transform the R strain into a pathogenic form.
Step-by-step explanation:
Fred Griffith discovered a phenomenon called transformation in the bacterium Streptococcus pneumoniae. Transformation is a process in which external DNA is taken up by a cell, changing its morphology and physiology. Griffith's transformation experiments involved two strains of this bacterium: a non-pathogenic rough (R) strain with a rough appearance due to the absence of a capsule, and a pathogenic smooth (S) strain that appears smooth under a microscope due to the presence of a capsule. He found that when he injected mice with a combination of live R strain and heat-killed S strain, the mice died, and only the S strain was recovered from the dead mice, proving that the R strain had been transformed into the S strain by what he called the "transforming principle." This transforming principle was later identified as DNA.