The problem. By the 1940s genes were known to be real, Mendel’s units, mapped onto chromosomes, but nobody knew what they were made of. The smart money was on protein, with its twenty-letter alphabet; DNA, with only four bases, looked too monotonous to carry the information of life.

The idea. Avery’s group built on Griffith’s observation that a harmless pneumococcus could be “transformed” into a virulent one by something from dead virulent cells. They purified that something and asked, chemically, what it was. Destroy the protein, transformation persists. Destroy the RNA, persists. Destroy the DNA with DNase, it stops. The transforming principle was DNA. The logic is a model of elimination: remove each candidate until only one survives.

Why it matters. This is the hinge of molecular biology, the moment heredity became a chemistry problem, setting up Watson and Crick’s structure nine years later. It’s also a masterclass in experimental design: a clean, subtractive test that answers a question the field had been guessing at. The rigor is the lesson as much as the result.

Verdict. A foundational experiment whose caution now reads as prescient, the authors were careful, almost understated, and the field was slow to accept it. Read it for how decisively a well-designed subtraction can settle a question. It hands the baton straight to the double helix.