A reasonable doubt
It seems obvious today that DNA carries heredity, but for a long time most scientists bet on protein instead. Their reasoning was sensible: proteins are built from twenty different amino acids, while DNA has only four bases. A four-letter alphabet looked too simple to encode the dazzling complexity of life, whereas proteins seemed rich enough. Settling the question took clever experiments, not assumptions.
Three experiments that settled it
- Griffith (1928): mixing harmless live bacteria with dead disease-causing ones somehow turned the harmless ones deadly — and the change was inherited by their descendants. Some 'transforming principle' had passed between them, though Griffith could not say what it was.
- Avery, MacLeod & McCarty (1944): they purified the transforming principle and destroyed each candidate in turn. Destroying protein or RNA changed nothing; destroying DNA abolished the effect. The transforming principle was DNA.
- Hershey & Chase (1952): using viruses that infect bacteria, they tagged protein and DNA with different radioactive labels. Only the DNA label entered the bacteria and directed new viruses. This convinced the remaining doubters that DNA, not protein, is the hereditary material.
The structure that explained heredity
Knowing DNA was the material still left a puzzle: how could a molecule both carry information and copy itself? The answer came in 1953, when Watson and Crick proposed the double helix, drawing on X-ray images from Rosalind Franklin and Maurice Wilkins and on Chargaff's rules. The structure was not just beautiful — it was self-explaining.
The famous understatement in their paper noted that the specific pairing they proposed 'immediately suggests a possible copying mechanism.' Because base pairing makes the strands complementary, separating them gives two templates, each able to rebuild its partner. Information storage and faithful copying — the two demands on any hereditary molecule — fell out of one elegant shape.