Avery-MacLeod-McCarty Experiment

• Extracted components from heat-killed S bacteria.
• After each extraction, S cells were mixed with R bacteria.
• R bacteria transformed each time until DNA was extracted from S cells.
• Avery and his colleagues concluded that DNA was the “transforming principle.”

 In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty provided additional experimental evidence using test tubes to strengthen Griffith’s “transforming principle.” Like Griffith, Avery and his colleagues used harmless R bacteria to determine the genetic factor of bacteria. First, they lysed heat-killed S cells extracted from Streptococcus pneumonia. When the lysate combined with R bacteria, virulent S bacteria were produced. To determine the factor responsible for transformation, Avery, MacLeod, and McCarty removed the sugar coats, proteins, RNA, and DNA from the lysate. The R bacteria remained non-virulent only when the DNA was removed from the lysate. In all other cases, the R bacteria were transformed. This experiment showed that DNA was the “transforming principle.”