Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types
The experiment that proved the gene is made of DNA, not protein.
For decades "the gene" was an abstraction. This experiment finally caught it in a test tube — and discovered it was made of DNA.
The big idea
Living things pass traits to their offspring through genes, but in the 1940s nobody knew what a gene was physically made of. The smart money was on protein: chromosomes contain both protein and DNA, and proteins are wonderfully varied, while DNA — built from just four repeating units — looked too simple to hold life's instructions.
Avery, MacLeod and McCarty settled it. Working with pneumonia bacteria, they purified the one substance that could carry an inherited trait from a dead cell into a living one, and ran it through chemical and enzyme tests. The answer was unambiguous: the carrier was DNA. The "too simple" molecule was the genetic material after all.
How it came about
Their starting point was a strange result from 1928. The British bacteriologist Frederick Griffith found that harmless pneumococci could be turned permanently deadly simply by mixing them with the heat-killed remains of a deadly strain. Something had passed from the dead bacteria to the living ones and rewritten their heredity. Griffith called it the "transforming principle" but had no idea what it was.
At the Rockefeller Institute in New York, Oswald Avery — a meticulous, soft-spoken researcher then in his sixties — spent more than a decade hunting that something. With his younger colleagues Colin MacLeod and Maclyn McCarty, he purified it, tested it, and tried every way to prove it wrong. When enzymes that chew up protein and RNA left it working, and only an enzyme that destroys DNA switched it off, the conclusion was inescapable. Avery, ever careful, wrote the result in cautious language — and the world was slow to believe him. He was never awarded the Nobel Prize.
Why it mattered
If genes are made of DNA, then heredity is chemistry — something you can isolate, analyse, and one day read and rewrite. This single finding pointed biology straight at the molecule and turned the structure of DNA into the most important open question in science. Five years later it would be answered by the double helix.
A way to picture it
Imagine a library burns down, but its complete catalogue survives in the ashes — and any nearby library that absorbs that catalogue suddenly reshelves itself to match. Which scrap of the wreckage carries the catalogue? You test by destroying things one at a time. Burn the leather bindings (the protein): the magic still works. Shred the index cards of one colour (the RNA): still works. Only when you dissolve the paper the catalogue is printed on (the DNA) does the power vanish. That paper is the gene.
Where it sits
Mendel (1866) showed that traits travel in discrete packets; this paper revealed what those packets are made of; Hershey and Chase confirmed it with viruses in 1952; and Watson and Crick's double helix in 1953 finally showed how a DNA molecule could store and copy the message. Avery's experiment is the quiet hinge in that chain — the moment "the gene" stopped being an idea and became a chemical you could hold.
The active fraction … consists principally, if not solely, of a highly polymerized, viscous form of desoxyribonucleic acid.
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…
The evidence presented supports the belief that a nucleic acid of the desoxyribose type is the fundamental unit of the transforming principle of Pneumococcus Type III.