Here's a breakdown:
* Frederick Griffith (1928) conducted the initial experiments that demonstrated bacterial transformation. He worked with two strains of *Streptococcus pneumoniae*: a virulent strain (smooth, or S) and a non-virulent strain (rough, or R). He showed that heat-killed S bacteria could transform live R bacteria into virulent S bacteria, indicating that some genetic material was transferred from the dead S cells to the live R cells.
* Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) followed up on Griffith's work. They performed a series of experiments to identify the transforming principle. They isolated and purified different components of the S cells (proteins, lipids, carbohydrates, and nucleic acids). They found that only DNA could transform R cells into S cells, proving that DNA is the genetic material.
Therefore, while Griffith's work laid the foundation for the discovery of bacterial transformation, Avery, MacLeod, and McCarty's experiments conclusively identified DNA as the transforming principle.
