Final answer:
Avery, MacLeod, and McCarty's experiments demonstrated that DNA was the transforming material by using enzymes to degrade different components (DNA, RNA, proteins) and observing that only when DNA was destroyed did the transformation capacity of the bacterial extract disappear.
Step-by-step explanation:
The foundational work by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944, which built upon Frederick Griffith's initial discovery of transformation, sought to identify the nature of the genetic material. Avery et al. employed rigorous experimental methods to ascertain that DNA was indeed the 'transforming material'. Isolating the smooth (S) strain Streptococcus pneumoniae from dead mice, they systematically treated these bacterial extracts with enzymes that destroyed specific biochemical components, such as proteins, RNA, and DNA. When the DNA of the S strain was degraded, the resulting mixture lost its ability to transform nonvirulent rough (R) strain bacteria into the virulent S form.
Conversely, if proteins or RNA were eliminated, the transformation still occurred, suggesting that these molecules were not the transforming principle. Their study was meticulous and controlled, using only one type of enzyme at a time and carefully observing the outcome of each treatment. The clear result was that without DNA, transformation did not occur, which led to the groundbreaking conclusion that DNA had to be the substance responsible for hereditary change.