In the early 1940s, the identity of the molecule carrying hereditary information remained unknown. Most scientists believed that proteins, due to their complex structures, were the only molecules capable of storing the vast amount of data required for life. Working at the Rockefeller Institute, Oswald Avery, Colin MacLeod, and Maclyn McCarty sought to identify the chemical substance responsible for transferring genetic traits in bacteria. Their research focused on bacterial transformation, a phenomenon previously observed but never chemically explained. Their systematic experiments aimed to isolate the substance of heredity and provided the first definitive evidence that the genetic material was deoxyribonucleic acid, or DNA, not protein.
The Precursor: Griffith’s Transforming Principle
The foundation for this inquiry was laid by the British bacteriologist Frederick Griffith in 1928, whose experiments with Streptococcus pneumoniae first demonstrated genetic transfer. Griffith studied two strains of the bacteria: a virulent S strain (“smooth” colonies with a capsule) and a non-virulent R strain (“rough” colonies lacking a capsule). Injecting mice with either the live R strain or the heat-killed S strain alone was harmless.
The surprising result occurred when Griffith injected mice with a mixture of the live R strain and the heat-killed S strain. The mice died, and living S strain bacteria were recovered from their blood. This outcome indicated that some substance from the dead S cells had been transferred to the live R cells, permanently changing them into the disease-causing S form. Griffith named this heritable agent the “Transforming Principle,” but its molecular nature remained unknown. Avery, MacLeod, and McCarty took on the challenge of isolating and chemically characterizing this transforming agent.
Methodology: Isolating the Genetic Material
Avery and his colleagues began by carefully preparing a cell-free extract from large cultures of heat-killed S-strain bacteria. This extract contained all cellular components, including proteins, lipids, RNA, and DNA. The team sequentially purified this crude mixture, using chemical processes to remove lipids and carbohydrates, leaving a solution rich in proteins, RNA, and DNA. The critical step involved treating the extract with highly specific enzymes designed to destroy one type of molecule at a time. They prepared separate batches and subjected each to a different enzyme before testing its ability to transform the R strain bacteria.
Enzyme Testing
Treating the extract with protease, an enzyme that breaks down protein, did not prevent transformation from occurring. Similarly, using ribonuclease (RNase), which degrades RNA, also failed to stop the R strain from acquiring the S-strain characteristic. This eliminated both protein and RNA as candidates for the hereditary molecule. The final test involved deoxyribonuclease (DNase), an enzyme specifically chosen to break down DNA.
The Definitive Conclusion: DNA is the Transforming Agent
When the transforming extract was treated with DNase, the ability to convert the R strain into the S strain was completely lost. This provided strong evidence that the substance necessary for transformation was DNA. Genetic information could not be transferred only when the DNA component was enzymatically destroyed.
The researchers also performed chemical analyses on the highly purified transforming substance. They determined that the substance had an elemental composition, including a specific nitrogen-to-phosphorus ratio, which closely matched known values for DNA. Furthermore, the purified substance lacked sulfur, a common element in many proteins, confirming that protein contamination was not responsible for the observed effect.
The substance also exhibited an ultraviolet absorption spectrum with a peak at 260 nanometers, a characteristic signature of DNA. These combined lines of evidence—enzymatic destruction and chemical composition—led Avery, MacLeod, and McCarty to conclude in their 1944 paper that the hereditary material responsible for the stable, heritable change was DNA.
Immediate Impact and Legacy
The publication of the experiment’s findings in 1944 was initially met with skepticism from the scientific community. Many prominent biologists remained reluctant to abandon the long-held belief that proteins were the carriers of genetic information. The idea that DNA, considered a relatively simple, repetitive molecule, could hold the complexity of heredity seemed improbable.
Independent confirmation was required before the new paradigm was fully embraced. This validation arrived in 1952 with the Hershey-Chase experiment, which used bacteriophages and radioactive labeling to confirm that DNA, not protein, was injected into bacteria during infection. This follow-up study provided the necessary reinforcement for Avery’s earlier work, shifting scientific opinion.
The collective evidence from Avery, MacLeod, and McCarty, followed by Hershey and Chase, established DNA as the universal molecule of heredity. Their work served as the intellectual launchpad for the field of molecular biology. The understanding that DNA was the genetic material allowed James Watson and Francis Crick to successfully determine the double helix structure of DNA in 1953, ushering in the modern era of genetics.