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.