Introduction
You want to understand how life replicates, but biological inheritance looks like an impenetrable mystery. Scientists long debated whether complex proteins or obscure nucleic acids carried the hereditary code. You face a foundational puzzle in twentieth-century biology: discovering the exact physical structure that makes heredity possible.
James D. Watson and Francis Crick solved this puzzle by decoding the molecular structure of deoxyribonucleic acid. The book reveals how intense ambition, fierce competition, and empirical clues drove the breakthrough at Cambridge University.
The following sections explore the scientific landscape, early failures, crucial experimental data, and the final double helix model. You will discover answers to specific historical questions.
- Which experiments proved that nucleic acids carry genetic information?
- How did rivalries between Cambridge and King's College shape the research?
- What role did X-ray diffraction data play in revealing the structure?
1. Background and Scientific Landscape
You find that early genetics focused almost entirely on proteins while ignoring deoxyribonucleic acid. Erwin Schrödinger published What Is Life? in 1944. You read the book in 1946 and learned that genes are the key components of living cells. O.T. Avery showed at the Rockefeller Institute that hereditary traits could be transmitted by purified DNA molecules, suggesting DNA was the true substance of genes.
Early experiments by Schrödinger and Avery established DNA as the primary candidate for genetic material, setting the stage for structural investigations.
Max Delbrück and Salvador Luria led the phage group during the decade from 1940 to 1950. You worked with Ole Maaløe and finished a set of experiments on the fate of a bacterial-virus particle inside a bacterium within three months. You discovered Maurice Wilkins showing an X-ray diffraction picture of DNA at the Naples meeting and later sitting to eat lunch with your sister, Elizabeth.
The shift from proteins to DNA
Sir Lawrence Bragg, born in 1890, received a Nobel Prize in 1915 with his father William Henry. You arrived at the Cavendish Laboratory in the fall of 1951. Sir Lawrence Bragg visited Max Perutz's office to discuss recently accumulated X-ray data and attempted to interpret them. Max Perutz came to England in 1936.
Francis Crick was 35 years old when you arrived at the Cavendish Laboratory. You watched Crick flood the corridor outside Sir Lawrence Bragg's office twice by neglecting to fasten securely the rubber tubing around his suction pump. Linus Pauling was 50 years old when he was expected to try for the DNA structure. Linus Pauling kept his model hidden behind a curtain until near the end of his lecture, then proudly unveiled the alpha-helix.
The Cavendish Laboratory and King's College rivalry
Maurice Wilkins received a letter from Linus Pauling asking for a copy of the crystalline DNA X-ray photographs. Rosalind Franklin was 31 years old and claimed DNA as her own independent problem rather than working as Maurice Wilkins's assistant. You received a check for travel expenses from Washington with an amount of 200 dollars.
You received a fellowship stipend of 3000 dollars for being in Copenhagen and had 1000 dollars of remaining funds after expenses for a year's stay in Cambridge. Herman forwarded the depressing news that the Fellowship Board would not approve your transfer to Cambridge.
2. Early Models and Setbacks
You find that Linus Pauling discovered the alpha-helix by relying on simple laws of structural chemistry and physical molecular models rather than complex mathematical equations. The key to Pauling's success was asking which atoms like to sit next to each other, using toy-like preschool molecular models instead of pencil and paper to test spatial configurations. Pauling successfully deduced the alpha-helix structure of polypeptide chains using physical models and basic chemical rules.
Initial attempts at building a three-chain DNA model failed due to incorrect chemical assumptions and insufficient water content.
Methodology of molecular modeling
Watson and Crick initially assumed that DNA possessed a very regular sugar-phosphate backbone located at the center of a helical three-dimensional configuration. By assuming the backbone was extremely regular and identical for all nucleotides, they could search for a helical configuration that allowed all backbone groups to share identical chemical environments. Alexander Todd's organic chemistry lab thought internucleotide bonds were phosphodiester bonds, supporting the idea of a regular backbone. Francis Crick and Bill Cochran successfully derived a general mathematical theory for the diffraction of X-rays by helical molecules. After finding flaws in a paper sent by V. Vand, Crick worked through equations with crystallographer Bill Cochran and derived a correct theory confirmed by Max Perutz's X-ray diagrams of the alpha-helix. Crick and Cochran published their successful mathematical theory of helical diffraction in Nature.
Rosalind Franklin insisted that DNA structures must be established purely through careful crystallographic analysis rather than speculative model-building. Franklin viewed molecular modeling as a last resort, arguing that hard facts would emerge only through patient collection of quantitative X-ray diffraction data. At her mid-November seminar, 15 people attended to hear Franklin present her preliminary crystallographic findings on DNA without mentioning molecular models.
The flawed three-chain structure
Watson and Crick's initial three-chain DNA model with a central sugar-phosphate core failed because it violated chemical bond limits and lacked sufficient water. When Rosalind Franklin, Maurice Wilkins, and their colleagues visited Cambridge to inspect the model, Franklin pointed out that the structure contained at least ten times too little water and relied on implausible magnesium ions. Franklin aggressively criticized the use of Mg++ ions to bind phosphate groups, noting they would be surrounded by tight shells of water molecules. The crystallographic repeat distance along the helical axis observed in Maurice and Rosy's pictures measured 28 Angstroms. The actual DNA water content exceeded what was built into Watson and Crick's early model by a factor of 10 times.
Sir Lawrence Bragg temporarily banned Watson and Crick from working on DNA after an academic dispute and mounting frustration with Crick's behavior. Following a confrontation over hemoglobin research and an embarrassing visit from the King's College group over the flawed DNA model, Bragg instructed Max Perutz that Watson and Crick must stop working on DNA. Crick accused Bragg of ignoring his ideas on hemoglobin shape, leading to a severe breach in their professional relationship. Bragg felt no fundamental scientific value had emerged from Crick's non-stop talking during a period of 35 years.
Watson faced the revocation and subsequent shortening of his American research fellowship due to violating terms by moving from Copenhagen to Cambridge. The National Research Council revoked his original twelve-month fellowship for not going to Stockholm, replacing it with an eight-month award terminating in May. Watson received a letter in January stating he was sacked from his original fellowship terms but awarded a revised two-thousand-dollar grant. The truncated duration of Watson's revised fellowship ended in the middle of May, accompanied by a financial penalty of 1000 dollars that Watson incurred for failing to follow the Board's advice to go to Stockholm.
3. Crucial Clues and Theoretical Breakthroughs
Chargaff's rules, Griffith's quantum calculations, and Franklin's B-form X-ray data provided the necessary constraints for the correct structure.
Watson studies the tobacco mosaic virus to master helical diffraction theory and find tactical cover for investigating nucleic acids. Hugh Huxley teaches Watson how to tilt the oriented TMV sample at several angles to reveal the helical pattern. Using a powerful rotating anode X-ray tube in the Cavendish, Watson obtains clear helical X-ray reflections of TMV. The molecular weight of TMV is 40 million, with a 23 angstrom turn along the helical axis, a 25 degree tilt of the sample, and 20 times faster picture-taking with the supertube.
Al Hershey and Martha Chase establish that infection of a bacterium by a phage involves the injection of viral DNA while very little protein enters the host. At an Oxford meeting with 400 microbiologists in the audience, Watson reads a long letter from Al Hershey detailing these phage experiments to an indifferent audience. Erwin Chargaff discovers that in all DNA preparations, the number of adenine molecules equals thymine, and guanine equals cytosine, with proportions varying by biological origin. During a dinner at Peterhouse arranged by John Kendrew, Chargaff scorns Watson and Crick for their lack of precise chemical knowledge.
Chargaff's base ratios
Theoretical chemist John Griffith performs semirigorous quantum-mechanical calculations at Crick's prompting, hinting that adenine sticks to thymine and guanine to cytosine by their flat surfaces. Crick discovers Griffith's calculations when visiting his rooms in Trinity, finding him with a girl and writing the pairs on the back of an envelope. Watson organizes biochemical evidence to conclude that DNA acts as a stable template for making RNA chains, which in turn serve as templates for protein synthesis. Watson tapes a paper sheet with the slogan DNA -> RNA -> protein on the wall above his desk in Clare College.
Cavalli-Sforza and Bill Hayes establish the existence of male and female bacterial sexes, where only a fraction of the male chromosomal material enters the female cell. Joshua Lederberg announced bacterial mating in 1946, and Watson now listens to 3 to 5 hours of nonstop talks by him at a microbial genetics meeting in Pallanza, later talking with Bill Hayes at Hammersmith Hospital. Linus Pauling proposes an erroneous three-chain DNA model with uncharged, hydrogen-bonded phosphate groups at the core. Pauling publishes a DNA model where phosphate groups contain bound hydrogen atoms instead of being ionized, violating basic nucleic-acid chemistry. Peter Pauling brings two copies of his father's manuscript to Cambridge, causing Watson and Crick to realize Pauling made a fundamental chemical blunder.
The B-form photograph
When DNA molecules are surrounded by high water content, they yield a distinct B-form X-ray pattern dominated by a clear black cross of reflections indicating helical symmetry. During a visit to King's College, Watson enters Rosalind Franklin's lab, views her B-form X-ray photograph shown by Maurice Wilkins, and immediately recognizes the simple helical parameters.
4. The Double Helix and Replication Mechanism
Placing the backbone on the outside and applying keto tautomeric forms revealed the complementary base pairs and the template replication mechanism.
Solving the backbone and tautomeric forms
You find that abandoning backbone-centered models solves previous stereochemical difficulties. Watson dismantles a repulsive backbone-centered molecule and spends days building backbone-out models with an angle of rotation of 30 to 40 degrees between adjacent bases. This allows the structure to twist into shapes compatible with the 34 Angstroms crystallographic repeat.
Watson initially attempts a like-with-like pairing scheme where identical bases pair together. This scheme collapses when American crystallographer Jerry Donohue points out that Watson has copied incorrect enol forms from J. N. Davidson's book.
When Watson explains his scheme in the office, Jerry Donohue protests that the enol tautomeric forms for guanine and thymine are wrong and that they occur in the keto form based on crystal structures like diketopiperazine.
Working alone in the office with cardboard base cutouts after Jerry Donohue's intervention, Watson suddenly realizes that A-T and G-C pairs form naturally with identical shapes. Adenine always pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine. This purine-to-pyrimidine pairing allows irregular sequences of bases to pack regularly in the center of the helix.
Francis Crick quickly pushes the bases together and confirms that no other arrangement satisfies Chargaff's rules while maintaining identical A-T and G-C shapes. Because adenine must always hydrogen-bond to thymine, and guanine must always hydrogen-bond to cytosine, the total amount of purine residues must equal the total amount of pyrimidine residues.
The complementary nature of the two intertwined chains immediately suggests a copying mechanism for genetic replication.
Watson lies awake with pairs of adenine residues whirling in front of his eyes, realizing the revolutionary implication that one chain serves as a template for the synthesis of the other. Because the base sequence of one chain automatically determines the sequence of its partner, gene replication can occur by separating the two identical chains and synthesizing two new daughter strands on the parental templates.
The publication of the structure
Max Perutz brings an MRC report containing Rosalind Franklin's and Maurice Wilkins' measurements with a 3.4 Angstroms meridional reflection and a 20 Angstroms helix diameter. Later, Rosalind Franklin freely shares her data and confirms that the sugar-phosphate backbone is on the outside.
When Francis Crick visits King's College, Rosalind Franklin shows him her data with obvious pleasure, exchanging hostility for a conversation between equals and confirming the backbone position. Meanwhile, Max Delbrück writes to describing how Linus Pauling gives a seminar on his DNA structure, which turns out to have impossible atomic contacts in its three-chained models.
After refining atomic coordinates and showing the model to Sir Lawrence Bragg, Maurice Wilkins, and Linus Pauling, Watson and Crick prepare a short manuscript for Nature. Watson's sister types the nine-hundred-word article starting with "We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA)," which is sent to Nature on the April 2 date of submission in 1953.
Conclusion
The discovery of the double-helical structure of DNA revealed the fundamental mechanism of genetic heredity and biological replication.
10 Key Ideas
- Early experiments by Schrödinger and Avery established DNA as the primary candidate for genetic material, setting the stage for structural investigations.
- Initial attempts at building a three-chain DNA model failed due to incorrect chemical assumptions and insufficient water content.
- Rosalind Franklin insisted that DNA structures must be established purely through careful crystallographic analysis rather than speculative model-building.
- Chargaff's rules, Griffith's quantum calculations, and Franklin's B-form X-ray data provided the necessary constraints for the correct structure.
- Al Hershey and Martha Chase established that infection of a bacterium by a phage involves the injection of viral DNA while very little protein enters the host.
- Theoretical chemist John Griffith performed semirigorous quantum-mechanical calculations at Crick's prompting, hinting that adenine sticks to thymine and guanine to cytosine by their flat surfaces.
- Placing the backbone on the outside and applying keto tautomeric forms revealed the complementary base pairs and the template replication mechanism.
- American crystallographer Jerry Donohue pointed out that Watson had copied incorrect enol forms from J. N. Davidson's book.
- Adenine always pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine.
- The discovery of the double-helical structure of DNA revealed the fundamental mechanism of genetic heredity and biological replication.