Discovering the double helix

No one could look at DNA and see a double helix. Its structure was worked out in the early 1950s by combining three kinds of evidence: patterns from X-rays, measurements of base amounts, and physical models.

Franklin’s X-ray patterns

When X-rays pass through a substance, its atoms scatter them. If the substance has a regular, repeating structure, the scattered X-rays form a pattern of spots, called a diffraction pattern. Mathematical analysis of that pattern reveals how the atoms are arranged in three dimensions. The technique is X-ray diffraction.

Rosalind Franklin produced the clearest X-ray patterns of DNA fibers. They showed that:

• DNA is a helix.
• Its diameter is uniform and too wide for a single strand, so it must have more than one.
• The strands are about 2 nm apart, which matches a purine pairing with a pyrimidine.
• A particular number of nucleotides fits in each turn.

Chargaff’s base counts

Erwin Chargaff measured how much of each base was present in DNA from many species. The percentages differed from species to species, but within each one, the amount of A nearly matched the amount of T, and the amount of G nearly matched C. In human DNA, for example, A and T are each about 30%, and G and C are each about 20%.

Chargaff didn’t know why, but the AT/GC rule explains it: every A is paired with a T, and every G with a C. It also means purines (A + G) make up half of the bases and pyrimidines (T + C) the other half.

Pauling’s model-building approach

Linus Pauling had worked out a common shape in proteins, the α helix, by building large models from ball-and-stick pieces. Physical models show whether atoms can actually fit together in a proposed three-dimensional structure. (Today researchers build such models on computers.) This approach proved to be the key to solving DNA.

Watson and Crick put it together

James Watson and Francis Crick assumed that nucleotides are linked in a line by the same kind of bond each time. Then they built ball-and-stick models that had to agree with all the known data.

Their first attempts failed. One model put the bases on the outside of the helix. Another paired each base with a copy of itself (A with A, G with G, and so on). But a purine–purine pair was too wide and a pyrimidine–pyrimidine pair too narrow to fit Franklin’s uniform diameter.

The breakthrough came when they saw that an A–T pair has nearly the same shape as a G–C pair. With those pairs on the inside and the backbones on the outside, every atom fit. They published the double helix in 1953.

Check yourselfWhy couldn’t the DNA double helix be built from A–G and T–C pairs?

A–G joins two double-ring purines, which would be too wide, and T–C joins two single-ring pyrimidines, which would be too narrow. Franklin’s data showed a uniform diameter, which only purine–pyrimidine pairs (A–T and G–C) can produce.