The Molecule of Heredity
By the late 1940s, biologists knew that genes — the units of heredity — resided on chromosomes, and chromosomes were composed of protein and deoxyribonucleic acid (DNA). The question was which one carried genetic information. The answer came in 1944 when Oswald Avery and colleagues demonstrated that DNA, not protein, was responsible for transforming bacteria — a result greeted initially with widespread skepticism, then grudging acceptance.
If DNA carried genetic information, its structure must somehow encode that information. Discovering how was the central problem in molecular biology in the early 1950s. The solution would require not just clever biology but first-rate chemistry, X-ray crystallography, and — controversially — a degree of collaboration and competition that still generates ethical debate.
The Structure of DNA: What Was Known
DNA's chemical composition was established by Phoebus Levene in the 1920s and refined subsequently. DNA consists of:
- A deoxyribose sugar (5-carbon ring, lacking the 2'-OH group present in RNA)
- A phosphate group (PO₄³⁻) linking successive sugars
- One of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C)
These three components link to form a nucleotide, and nucleotides connect into a long chain — the DNA backbone — with bases pointing outward from the sugar-phosphate spine.
Erwin Chargaff (Columbia University) made a crucial discovery in 1950: the base composition of DNA varied between species, but within any species, the amount of adenine equaled the amount of thymine (A=T), and the amount of guanine equaled the amount of cytosine (G=C). These "Chargaff's rules" were a major clue — but Chargaff himself did not immediately see their structural implication.
The Players in the Race
Four groups were working toward DNA's structure in the early 1950s:
Linus Pauling (Caltech), the most celebrated chemist of his era, who had just solved the structure of the alpha helix (a common protein structure) using model-building and his chemical intuition. Pauling's approach: propose chemically reasonable structures, build models, test against X-ray data.
Maurice Wilkins and Rosalind Franklin (King's College London), experts in X-ray crystallography. Franklin, a meticulous experimentalist, was generating the best X-ray diffraction images of DNA crystals yet obtained.
James Watson and Francis Crick (Cambridge), a young American biologist and a British physicist turned structural biologist, working at the Cavendish Laboratory. Watson and Crick were also model-builders.
Rosalind Franklin and Photo 51
Rosalind Franklin (1920–1958) brought skills from X-ray crystallography of coal to bear on DNA fibers. By carefully controlling humidity, she distinguished two forms of DNA: A-form (crystalline, observed at low humidity) and B-form (extended, observed at high humidity in the presence of water). The B-form was the biologically relevant one.
In May 1952, Franklin captured what became known as "Photo 51" — an X-ray diffraction image of B-form DNA fibers. The image showed a clear X-shaped diffraction pattern, indicative of a helical structure. Careful analysis of the image's spacing indicated:
- The helix had a pitch (rise per complete turn) of 3.4 nm
- The helix had 10 base pairs per turn
- The helix diameter was approximately 2 nm
Franklin also calculated that the phosphate groups were on the outside of the helix, not the inside — a critical structural detail.
In January 1953, Maurice Wilkins showed Photo 51 to James Watson without Franklin's knowledge. Watson immediately recognized its significance. He reported the key measurements to Crick.
The ethics of this episode have been debated for decades. Franklin did not give permission for her data to be shared. Watson and Crick later acknowledged that Photo 51 was critical to their model, but the extent of that acknowledgment in the original 1953 paper was limited.
Linus Pauling's Near Miss
In February 1953, Pauling and his son Peter published a proposed DNA structure — a triple helix with the phosphate groups on the inside. Watson and Crick immediately identified a fatal flaw: placing phosphates on the inside without counterions would create enormous electrostatic repulsion; the structure couldn't hold together. Pauling, working without access to Franklin's crystallographic data (partly due to Cold War travel restrictions), had made an error that his own chemical expertise should have prevented.
The miss gave Watson and Crick urgency: Pauling would correct his model quickly. They needed to move fast.
The Double Helix
Working with their cardboard and metal molecular models, Watson made the critical chemical insight. He had been trying to fit like-with-like base pairings (A with A, G with G). Then he tried complementary pairing: A with T, G with C. The geometry fit perfectly.
An A-T base pair is held together by two hydrogen bonds. A G-C base pair is held together by three hydrogen bonds. Both base pairs span the same distance across the helix (1.08 nm), which explains why the helix has a uniform diameter.
The structure clicked into place. DNA is a double helix: two antiparallel chains wound around a central axis, with:
- Sugar-phosphate backbones on the outside
- Complementary base pairs on the inside, held together by hydrogen bonds
- A right-handed helical twist, completing one turn every 3.4 nm (10 base pairs × 0.34 nm per base pair)
Watson and Crick submitted their landmark paper to Nature on April 2, 1953. Published April 25, 1953, it is one of the most famous papers in scientific history — just over a page long, with the famous understatement: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
That mechanism was semiconservative replication: when DNA replicates, each strand serves as a template for a new complementary strand. The sequence of bases — the genetic code — is preserved through the complementary pairing rules. This was confirmed experimentally by Meselson and Stahl in 1958.
The Nobel Prize and Franklin's Legacy
In 1962, Watson, Crick, and Wilkins received the Nobel Prize in Physiology or Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material."
Rosalind Franklin had died of ovarian cancer in 1958, at age 37. Nobel Prizes are not awarded posthumously. Whether she would have shared the prize had she lived is a question that cannot be answered — but her contribution to the discovery is now widely recognized as having been underacknowledged in her lifetime.
Franklin went on to do important work on RNA and tobacco mosaic virus before her death. Her X-ray crystallographic techniques were central to structural biology.
Chemical Implications
The double helix structure had immediate and profound chemical implications:
Genetic code: The sequence of bases along one strand encodes genetic information (later deciphered as codons — triplets of bases specifying amino acids). The code is stored in chemistry.
DNA replication: The enzyme DNA polymerase reads a template strand and synthesizes a complementary copy, always pairing A with T and G with C. Replication chemistry is the chemistry of complementary base pairing.
Transcription and translation: DNA is transcribed into RNA (which uses uracil in place of thymine), which is translated into protein. The central dogma of molecular biology is fundamentally a story of chemical information transfer.
Mutation: Changes in the base sequence — substitutions, insertions, deletions — alter the genetic code. Chemistry underlies both mutation and repair.
The discovery of DNA's structure launched molecular biology as a discipline and directly enabled genetic engineering, PCR (polymerase chain reaction), DNA sequencing, CRISPR-Cas9 gene editing, and genomic medicine — arguably the most consequential suite of technologies arising from any single structural discovery in science.