GENETICS

Avery and the Role of DNA

1944 to 1952 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

Avery, MacLeod and McCarty showed in 1944 that the substance able to change one type of bacterium into another was DNA, a result that many researchers resisted for years.

DNA is the molecule that stores hereditary information, and two experiments established this role. A substance extracted from dead bacteria could permanently change the type of living ones, and after years of purification Avery, MacLeod and McCarty identified the substance as DNA. Many researchers resisted the result. Proteins were varied and chemically interesting, whereas DNA was four units repeated, and so monotonous a molecule seemed an implausible place to store such information. Eight years later Hershey and Chase labelled the protein and the DNA of a virus with different radioactive isotopes, forms of an atom that can be traced by their radiation. They showed that only the DNA entered the cell that the virus infects, and that the protein coat remained outside.

Why it matters

Had researchers kept protein as the molecule of heredity, fifty years of biology would have been directed towards the wrong molecule. The objection to DNA was an argument from plausibility, which judges a result by what seems likely: DNA appeared too simple to store information. That simplicity is precisely what makes DNA copyable.

Dating & uncertainty

Many researchers doubted the 1944 experiment on transformation, the change of one type of bacterium into another, because they expected protein to be the molecule of heredity. The 1952 experiment used a phage, a virus that infects bacteria, and followed its DNA and its protein coat separately. It convinced almost every researcher, although its own authors cautioned against reading too much into it.

Earth history as one day

23:59:59.99841.6 ms before midnight

if Earth’s 4.54 billion years were compressed into a single day