Oswald Avery’s experiment, published in 1944, demonstrated that DNA is the molecule responsible for heredity. Working with colleagues Colin MacLeod and Maclyn McCarty at Rockefeller University, Avery isolated the substance that could permanently transform one type of bacteria into another and proved it was DNA, not protein as most scientists assumed. The paper, “Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types,” is now considered one of the most important publications in 20th-century biology.
The Mystery Avery Set Out to Solve
Avery’s work built on a puzzling discovery made 16 years earlier by British bacteriologist Frederick Griffith. In 1928, Griffith was studying pneumonia-causing bacteria (pneumococcus) that came in two forms: a virulent “smooth” (S) strain with a polysaccharide capsule coating its surface, and a harmless “rough” (R) strain that lacked this capsule. Mice injected with the S strain died within days. Mice injected with the R strain survived.
Griffith’s key finding came when he injected mice with a mixture of heat-killed S bacteria and live R bacteria. Neither component should have been dangerous on its own: the S bacteria were dead, and the R bacteria were harmless. Yet the mice died. When Griffith examined their blood, he found live S-type bacteria with intact capsules. Something from the dead S bacteria had permanently transformed the living R bacteria into a lethal form, and this change passed to future generations of the bacteria.
Griffith called this unknown substance the “transforming principle,” but he never identified what it was chemically. That question sat open for years. Avery, who had spent decades studying pneumococcus at Rockefeller, took it on.
How Avery Identified the Transforming Substance
Avery’s approach was methodical elimination. He and his team grew large quantities of the smooth (S) strain of pneumococcus, broke open the cells, and then systematically removed one type of biological molecule at a time to see which substance carried the ability to transform harmless R bacteria into virulent S bacteria.
First, they stripped away the polysaccharide capsule material. Transformation still worked. Then they used enzymes to destroy any proteins in the extract. Transformation still worked. They removed lipids (fats). Still worked. At each step, the active substance survived, ruling out proteins, carbohydrates, and fats as candidates.
What remained was a highly purified substance that Avery’s team then subjected to every analytical method available at the time. They used ultracentrifugation, electrophoresis, and ultraviolet spectroscopy. The results were consistent: the active fraction contained no detectable protein, no unbound lipid, and no serologically reactive polysaccharide. It consisted principally, if not solely, of a highly polymerized, viscous form of DNA.
The Chemical Evidence for DNA
Avery’s team didn’t rely on a single test. They built a case from multiple lines of chemical evidence. One of the most compelling was elementary chemical analysis of the purified substance. Proteins contain relatively high amounts of nitrogen but very little phosphorus. DNA, by contrast, has a characteristic ratio of nitrogen to phosphorus close to 1.69. Four independent preparations of the transforming substance yielded nitrogen-to-phosphorus ratios ranging from 1.58 to 1.75, closely matching the theoretical value for DNA and inconsistent with protein.
The decisive confirmation came from enzyme tests. When the team treated the purified substance with enzymes that specifically destroy DNA, transformation was abolished. Enzymes that break down proteins or RNA had no effect. This was the strongest piece of evidence: the molecule responsible for hereditary transformation was specifically vulnerable to DNA-destroying enzymes and nothing else.
Why Many Scientists Didn’t Believe It
Despite the rigor of the work, Avery’s conclusion met significant skepticism. In the 1930s and 1940s, the scientific consensus held that genes were made of protein. Proteins were known to be enormously variable in structure, with 20 different amino acid building blocks that could combine in essentially infinite arrangements. This variability seemed like the only plausible explanation for the vast range of precise effects that genes could produce.
DNA, by comparison, was considered boring. It contained only four chemical bases, and a prevailing idea called the tetranucleotide hypothesis held that these four bases occurred in equal, repeating proportions, like a monotonous chain. If DNA was just a repetitive polymer, it couldn’t possibly carry complex genetic information. As late as 1936, the crystallographer Dorothy Wrinch voiced the predominant view when she suggested that genes were proteins interwoven in a DNA structure, with DNA serving as mere scaffolding.
There were also technical objections. Some critics argued that Avery’s DNA preparations might contain trace amounts of protein too small to detect, and that this contaminating protein could be the real transforming agent. Avery himself was cautious in his conclusions, writing that “if the results of the present study on the chemical nature of the transforming principle are confirmed, then nucleic acids must be regarded as possessing biological specificity.” He understood the weight of what he was claiming and left room for verification.
How the Discovery Was Eventually Confirmed
The tide turned gradually. In 1952, Alfred Hershey and Martha Chase performed an experiment using bacteriophages (viruses that infect bacteria) and confirmed that DNA, not protein, carried genetic information. Their work used a completely different experimental system and reached the same conclusion Avery had published eight years earlier. The Hershey-Chase experiment is often credited with convincing the remaining skeptics.
The following year, James Watson and Francis Crick determined the double-helix structure of DNA, revealing exactly how a molecule with only four bases could encode vast amounts of information through the sequence of those bases along the strand. This demolished the tetranucleotide hypothesis and vindicated Avery’s findings. The problem had never been that DNA was too simple to carry genetic information. Scientists had simply underestimated how much complexity could arise from varying the order of four repeating units along a very long chain.
Avery’s Place in Scientific History
Avery never received the Nobel Prize for his discovery, despite the paper now being regarded as a landmark in molecular biology. He was 67 when the paper was published and died in 1955, just two years after Watson and Crick’s structural work brought DNA to the center of biology. The Nobel committee’s records show he was nominated but never selected, partly because the significance of the finding wasn’t fully accepted during his lifetime.
What makes Avery’s experiment so foundational is not just the conclusion but the method. He demonstrated that a specific, identifiable chemical substance could produce a permanent, heritable change in a living organism. Before this work, nobody had connected a particular molecule to the concept of a gene. Avery’s paper drew a direct line between chemistry and heredity, opening the door to everything that followed in molecular genetics.