Watson & Crick
What shape is the blueprint of life?
That time, that place
Watson and Crick ran no experiments. What they did was build models.
Atoms and bonds cut from sheet metal and wire, fitted together and refitted. It was the method Pauling had used successfully on proteins.
Why this question
They had three things in hand.
First, Chargaff's odd rule: in the DNA of any organism, adenine and thymine come in equal amounts, and so do guanine and cytosine. Why, nobody knew.
Second, the measurements from Franklin's diffraction data.
Third, the idea of putting the bases on the inside rather than the outside.
What was found
Two strands wind into a helix with the bases paired on the inside. Adenine meets only thymine, guanine only cytosine, because only those pairings fit the sizes and bond angles. Chargaff's rule falls out on its own.
But the real beauty lies elsewhere.
Since the strands are complementary, one strand determines the other completely. Pull the two apart and each becomes a template for rebuilding its partner.
Seeing the structure was seeing how it copies. The paper's last sentence read: it has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.
How the genetic material copies itself was unknown
A model matching the diffraction numbers and explaining Chargaff's rule as well
Molecular biology opened from here. Discussion of whose data it was built on continues to this day