(1) How was the structure of DNA discovered?

DNA (deoxyribonucleic acid) was first isolated, from white blood cells, by the Swiss scientist Friedrich Miescher in 1869.

Before the crucial X-ray photograph of Rosalind Franklin, chemical analysis showed that four bases seem to occur in ratios of 1 : 1 for one pair and 1 : 1 for another pair.

Adenine (A) with thymine (T)  was a 1 : 1 ratio and cytosine (C) with guanine (G) was also a 1: 1 ratio.

What were the roles of the scientists Watson, Crick, Franklin and Wilkins?

 

(a) In the 1950s, Rosalind Franklin working for Maurice Wilkins examined strands of DNA using a technique called X-ray diffraction analysis.

The sample under investigation, e.g. a DNA crystal strands, is bombarded with X-rays and the layers of atoms behave like a diffraction grating and scatter the X-rays in particular pattern that depends on the 3D arrangement of atoms in the molecule.

The path of the scattered X-rays is recorded on a photographic plate.

Complex mathematical analysis of the X-ray diffraction pattern, indicated a double helix structure.

Rosalind Franklin died tragically young from cancer, and never received the Nobel Prize she would have undoubtedly received, BUT, in one of the last things she wrote in her laboratory notebook, she speculated that DNA had a helix structure.

 

(b) Later Frances Crick and James Watson gathered together this X-ray data, Crick had access to Rosalind Franklin's 'classic' X-ray photograph of crystallised DNA, characteristic of a helical structure together with other information ...

including the chemical analysis of DNA, particularly the ratios of the four bases (adenine, cytosine, guanine and thymine) in which pairs always had a 1:1 ratio (A & T and C & G), plus the known shapes of the four base molecules ...

... and then built a model and deduced what we recognise today as the double helix structure of DNA - brilliant insight, both won the Nobel Prize along with Maurice Wilkins.

The important thing is that the experimental observations from chemical and structural analysis fitted the evidence based model devised by Crick and Watson.

Crick, Watson and Wilkins all received the Nobel Prize for their work on DNA.

Rosalind Franklin, Physicist and Biologist, tragically dying young from a combination of pneumonia and advanced cancer. All the other three scientists mentioned above received the ultimate accolade of a Nobel prize. Rosalind Franklin would also have received a Nobel prize, if she had not died so tragically young..

Key points in the DNA story

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

The discovery of DNA’s structure was a groundbreaking moment in biology, revealing how genetic information is stored and passed on.

Key Scientists Involved

• James Watson & Francis Crick – Developed the double helix model of DNA in 1953.

• Rosalind Franklin – Used X-ray diffraction to capture Photograph 51, which provided crucial evidence of DNA’s helical shape.

• Maurice Wilkins – Worked alongside Franklin and later shared her findings with Watson and Crick.

• Erwin Chargaff – Discovered that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G), known as Chargaff’s rules.

Crucial Scientific Evidence

• Photograph 51 – Franklin’s X-ray diffraction image showed DNA’s helical structure.

• Chargaff’s Rules – The ratio of bases suggested a base-pairing mechanism.

• Model Building – Watson and Crick used Franklin’s data to construct a 3D model, proving DNA is a double helix with complementary base pairs.

Other points about the DNA storyline

• DNA Structure:

  • Made of two strands forming a double helix.

  • Strands are held together by hydrogen bonds between base pairs.

  • Bases pair A-T and C-G.

• Discovery Timeline:

  • 1869 – DNA first identified.

  • 1943 – DNA confirmed as genetic material.

  • 1953 – Watson & Crick publish the double helix model.

• Importance:

  • Explains genetic inheritance.

  • Forms the basis for modern genetics and biotechnology.

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Summary of learning objectives and key words or phrases

How was the molecular structure of DNA investigated and worked out?

X-ray diffraction chemical analysis of bases.

Know about the contributions to research into the nature of DNA by Friedrich Miescher (discoverer), Rosalind Franklin and Maurice Wilkins (X-ray DNA crystal analysis) and finally, Frances Crick and James Watson who successfully proposed the correct double helix structure of DNA.

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