What if the carrier of heredity is not protein?

That time, that place

For sixteen years Griffith's something had no name. Most researchers assumed it was protein.

There were reasons. Protein is built from twenty amino acids strung together, so the combinations are effectively endless. DNA has only four bases, and a widely held view took those four to repeat in fixed proportion — far too monotonous a substance to hold information.

Why this question

Avery, MacLeod and McCarty went at it head-on: strip the active extract of one component at a time and see which removal stops the transformation.

An enzyme that cuts protein. Transformation proceeded.

An enzyme that cuts RNA. It proceeded.

An enzyme that cuts DNA. It stopped.

What was found

The carrier of heredity was DNA. This was 1944.

The paper's tone was remarkably cautious. Avery did not declare; he went no further than saying the substance appeared to carry the information. He was sixty-six, and not a man for large claims.

The field's response was lukewarm. The main objection was that no purification is perfect: if even a trace of protein remained in the extract, that trace could be the real agent. It was hard to answer.

Full acceptance came only after Hershey and Chase reached the same conclusion with viruses in 1952. Avery died in 1955 without a Nobel Prize.