1.5.8Biological Molecules

The Watson Crick Model — AQA A Level Biology 7402 Revision Notes & Topical Questions

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1.5.8 The Watson Crick Model — AQA A Level Biology 7402 notes: Explain how Watson and Crick's 1953 double-helix model was built from evidence and why the structure suggested a copying mechanism.By the early 1950s DNA was accepted as the genetic material, but how its structure cou…

Chemical analysis
X-ray diffraction
Existing knowledge of molecular bonding
double helix
peer review
Exam-style question 1

Section 1 of 9The question in 1950

By the early 1950s DNA was accepted as the genetic material, but how its structure could store information and be copied was unknown. Watson and Crick, working at Cambridge, built physical models of DNA to fit the available data. They did not do the key experiments themselves; they combined other scientists' evidence.

Section 2 of 9Evidence used

Evidence used

Chemical analysis

DNA contains phosphate, deoxyribose sugar and four bases. Chargaff showed that the amount of A equals T and G equals C, which suggested the bases paired.

X-ray diffraction

Rosalind Franklin and Maurice Wilkins fired X-rays at crystalline DNA fibres and recorded diffraction patterns on film.

Franklin's "Photo 51" showed an X-shaped pattern characteristic of a helix,
- and measurements suggested a regular repeating spacing and the diameter of the molecule.
  - (of the bases and of the turns)

Existing knowledge of molecular bonding

Existing knowledge of molecular bonding e.g. the shape of the bases and the possibility of hydrogen bonding between them.

Section 3 of 9The model

Watson and Crick proposed that DNA is a double helix of two antiparallel polynucleotide chains.

The sugar-phosphate backbones are outside; the bases point inwards, with a purine always pairing with a pyrimidine (A-T and G-C).

Using a purine-pyrimidine pair explained the constant width seen in the X-ray data.

Hydrogen bonds hold the strands together.

Their model also suggested a way to copy DNA: separating the strands would expose two templates, each able to direct the synthesis of a complementary partner, giving two identical molecules.

This prediction was later supported by Meselson and Stahl (1.5.6).

Watson, Crick and Wilkins received a Nobel Prize in 1962; Franklin had died in 1958.

Section 4 of 9Scientific method and its lessons

The case shows how science works

the model made predictions that could be tested;
- it explained existing data (Chargaff's ratios, X-ray pattern) and was later supported by new evidence (semi-conservative replication).

Peer review

Publication in the journal Nature allowed other scientists to evaluate it - peer review and repeatability are central.

It also shows how work depends on a community and on techniques, such as X-ray crystallography, and raises issues of credit and ethics, since Franklin's data were shared without her full knowledge.

A modern view

the structure is accurate in outline, but we now know DNA can adopt other forms and can be supercoiled.
Models in biology are therefore refined rather than abandoned, and exam answers should describe a model as supported by evidence rather than proven.

Linkbase-pairing rules from this model underpin replication (1.5.6), the base calculations in 1.5.7 and the code-reading in transcription and translation.

Section 5 of 9The role of other scientists and techniques

Franklin's work was crucial.

The role of other scientists and techniques

Using a very fine beam of X-rays directed at DNA fibres, she produced images in which the cross pattern showed a helix, and her measurements indicated that the phosphate groups were on the outside.

Chargaff's ratios gave Watson and Crick the idea of base pairing; Linus Pauling's work on the alpha-helix in proteins encouraged the idea of building models.

Watson and Crick cut out cardboard shapes of the bases and moved them until they found that A-T and G-C pairs had identical dimensions and fitted neatly between two backbones.

Later techniques such as X-ray crystallography of crystals, electron microscopy and DNA sequencing have confirmed and extended the model. This is a good example of a theory being accepted because it fits all the evidence available and generates testable predictions.

Section 6 of 9Answering questions on the history

Questions on this topic often ask you to explain why a particular piece of evidence was important, or why the model was accepted.

Answering questions on the history

Give the evidence, state what it showed, and say how it fitted the model.

For example: "Chargaff found equal amounts of A and T and of G and C, which suggested to Watson and Crick that A pairs with T and G with C".

Avoid vague statements such as "they discovered DNA", since DNA was already known; the contribution of the model was the three-dimensional structure and its explanation of copying.

Finally, remember the key vocabulary

antiparallel, complementary, purine, pyrimidine, hydrogen bond and phosphodiester bond.
Use them precisely in any question about the structure.

Section 7 of 9Exam-style questions

Explain how the data from Chargaff and from X-ray diffraction helped Watson and Crick to propose the structure of DNA. [4]4 marks
Why was the model of Watson and Crick accepted by other scientists? [3]3 marks

Section 8 of 9Key terms

Term	Meaning
X-ray diffraction	Technique using X-rays scattered by crystals to deduce molecular structure.
Double helix	Two strands twisted into a spiral.
Chargaff's data	Showed equal amounts of A and T and of G and C in DNA.
Peer review	Scrutiny of work by other scientists before publication.
Model	A representation used to explain data and make predictions.
Complementary	Fitting together in a specific way, as A with T and G with C.

Section 9 of 9Exam technique

Saying Watson and Crick did the X-ray experiments. The diffraction work was done by Franklin and Wilkins.

Forgetting that bases pair by purine-pyrimidine combinations, giving constant width.

Saying a model is proved true. Models are supported by evidence and may be modified.

Stating that the model was only about structure. It also suggested how copying would work.

Last updated: 18 October 2026