In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith's experiment and concluded that the transforming material was pure DNA, not protein or RNA. These investigators found that DNA extracted from a virulent strain of the bacterium Streptococcus pneumoniae, also known as pneumococcus, genetically transformed an avirulent strain of this organism into a virulent form.
Type IIR cells ⇒ No transformation
Type IIR cells + Type IIIS DNA extract ⇒ Transformation
Type IIR cells + Type IIIS DNA extract + DNase ⇒ No transformation
Type IIR cells + Type IIIS DNA extract + RNase ⇒ Transformation
Type IIR cells + Type IIIS DNA extract + Protease ⇒ Transformation