Three molecular biologists demonstrated that the genetic transformation of bacteria is caused by DNA, providing direct evidence about the chemical nature of hereditary information. Their discovery, doubted at first, eventually led geneticists to understand that DNA carried life’s genetic blueprints.
Until the Avery-MacLeod-McCarty experiment demonstrated that DNA is the hereditary chemical of life, most biologists believed that the substance responsible for heredity was protein because of its extensive diversity and variability. The Avery-MacLeod-McCarty discovery revolutionized the study of the biological sciences by focusing the study of living systems on molecular mechanisms and by changing the focus of the chemical nature of heredity from proteins to DNA. (http://0-search.ebscohost.com.usmalibrary.usma.edu/login.aspx?direct=true&db=ers&AN=89116362&site=eds-live)
In 1941, geneticist Maclyn McCarty extended MacLeod’s experiments by using an enzyme to digest the type III polysaccharide to remove it from the preparation. By early 1942, upon addition of alcohol to the preparation, a stringy, fibrous material precipitated. McCarty showed that all enzymes that degraded DNA destroyed the transforming principle, but inactivating these enzymes by heat eliminated their ability to destroy the transforming principle. By this time, the laboratory was convinced that the transforming and hereditary chemical was DNA
According to CPT Hummel, Dept. of Chemistry & Life Sciences, “The experiment by Avery, MacLeod, and McCarthy was pivotal in that it confirmed Frederick Griffith’s observation of transformation from the 1920s which had been scrutinized as potentially being contaminated. Cadets in the Introduction to Biology course look at both experiments to see how over 80 years ago scientists were just discovering the role of DNA as our genetic material. This experiment demonstrated that DNA was responsible for transforming the benign R type cells into the virulent S type.” Coincidentally, this week he was actually going over this experiment with the cadets, as they begin their genetics block.
LTC Goodin commented that in CH375, Intro to Biology, “We cover many different experiments that led to our current understanding of DNA as the genetic material. An entire block (11 lessons) of instruction is dedicated to the central dogma of molecular biology (DNA-RNA-Protein) and the relationship between genetics, heredity, and evolution.” He believes, “that one of the most interesting ways to learn about DNA and molecular biology is to follow the experiments that led to our current understanding. This is a big part of the biology course.”
The original article: Avery, Oswald T., Colin M. MacLeod, and Maclyn McCarty. “Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III.” Journal of Experimental Medicine 79, no. 2 (February, 1944): 137-158.
Contents contributed by Manja Yirka, Continuing Resources Librarian and Liaison Librarian to Department of Chemistry and Life Sciences.