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." 

Hershey-Chase Experiment:

 

Fig 2. Hershey- Chase experimented with radioactive phosphorous and sulfur to confirm that DNA is the “Transforming principle”. (Author: Thomasione. This is a file from the   Wikimedia Commons; GNU Free Documentation License)

• Labeled bacteriophage DNA with radioactive phosphorus
• After bacteriophage infected bacteria, phosphorus was found in bacteria
• Labeled protein coat with radioactive sulfur
• After bacteriophage infected bacteria, no sulfur was found in bacteria
• Hershey and Chase provided further evidence that DNA was the "transforming principle".

Although Avery, MacLeod, and McCarty discovered some evidence to show that DNA was responsible for the transfer of information, many people were still skeptical and believed it was protein. Alfred Hershey and Martha Chase were determined to provide more concrete evidence to prove that DNA was the genetic material in bacteriophages.

In their first experiment, Hershey and Chase labeled bacteriophage DNA by injecting radioactive phosphorus into the bacteriophage. Because DNA contains phosphorus and amino acids do not, only the DNA was tagged. After the bacteriophage infected a strain of E. coli, Hershey and Chase observed radioactive phosphorus in the bacteria.

In their second experiment, Hershey and Chase injected the bacteriophage with radioactive sulfur in order to tag only the protein coat. This time, after the bacteriophage infected the E. coli, Hershey and Chase did not observe the presence of sulfur in the bacteria.

From their experiments, Hershey and Chase concluded that DNA was responsible for transferring information in Griffith's experiment. Hershey and Chase's experiment finally convinced everyone of DNA's role as the genetic material in bacteriophages.