NCEA Level 2 · Year 12 · Biology · 2 minutes

How we discovered the shape of DNA

A blurry X, a twisted ladder, and a molecule that copies itself.

Key terms

Double helix
The twisted ladder shape of DNA, with sugar phosphate backbones as the sides.
Base pair
A rung of the DNA ladder made of two bases joined by weak hydrogen bonds.
Complementary base pairing
The rule that adenine pairs only with thymine and cytosine only with guanine.
Antiparallel
Running in opposite directions, as the two strands of DNA do.
Semi conservative replication
DNA copying in which each new molecule has one old strand and one new strand.
DNA polymerase
The enzyme that adds complementary nucleotides to each template strand when DNA is copied.

Transcript

0:00

May, nineteen fifty two, London. A thin fibre of DNA sits in an X ray beam for a total of sixty two hours. Out comes a blurry black X. So how does a blurry X reveal the shape of life's instruction manual?

0:18

The scientist was Rosalind Franklin, with her student Raymond Gosling. The image: Photograph fifty one. That cross of spots is the signature of a helix, seen from the side.

0:31

In January nineteen fifty three, her colleague Maurice Wilkins, born in Pongaroa, New Zealand, showed it to James Watson. Franklin didn't know. Watson and Francis Crick used the clue to build their double helix model, and Franklin's paper was published alongside theirs.

0:49

In nineteen sixty two, Watson, Crick and Wilkins shared a Nobel Prize. Franklin had died four years earlier, aged thirty seven, and a Nobel can't go to someone who has died.

1:01

So what is the double helix? Picture a twisted ladder. The sides are sugar phosphate backbones: deoxyribose sugar, phosphate, over and over. The rungs are base pairs, held by weak hydrogen bonds. And the two strands run in opposite directions: they're antiparallel.

1:20

The bases are picky. Adenine only pairs with thymine. Cytosine only pairs with guanine. A with T, C with G. That's complementary base pairing, so if you know one strand, you can work out its partner.

1:35

Watson and Crick saw the payoff. This pairing, they wrote, immediately suggests a possible copying mechanism. Helicase unzips the helix, breaking the hydrogen bonds. Each old strand becomes a template, and DNA polymerase adds complementary nucleotides. Result: two identical molecules, each with one old strand and one new. That's semi conservative replication.

2:01

So, three things. One: Photograph fifty one was the clue that DNA is a helix. Two: sugar phosphate backbones, joined by complementary bases, A with T, C with G. Three: each old strand is a template, so every new molecule is semi conservative.

Sources

Every fact in the video, and where it comes from.

  1. 1
    kcl.ac.uk/the-story-behind-photograph-51
    • Photograph 51 was taken at King's College London in May 1952 by Rosalind Franklin and her PhD student Raymond Gosling; the DNA fibre was exposed to X-rays for a total of 62 hours.
    • The X-shaped 'cross' of spots in Photograph 51 is the signature of a helix viewed from the side.
    • In January 1953 Maurice Wilkins showed Photograph 51 to James Watson; Franklin was unaware of this.
    • Watson and Crick's double-helix paper was published in Nature on 25 April 1953; Franklin and Gosling's paper with Photograph 51 appeared in the same issue.
  2. 2
    embryo.asu.edu/pages/photograph-51-rosalind-franklin-1952
    • Watson and Crick worked at the Cavendish Laboratory, Cambridge, and built a model of DNA.
  3. 3
    nature.com/articles/171737a0
    • Watson and Crick's double-helix paper was published in Nature on 25 April 1953; Franklin and Gosling's paper with Photograph 51 appeared in the same issue.
    • Watson and Crick wrote that the specific pairing they proposed 'immediately suggests a possible copying mechanism for the genetic material'.
    • The 25 April 1953 issue of Nature carried Watson & Crick 'Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid' (p. 737), Wilkins, Stokes & Wilson 'Molecular Structure of Deoxypentose Nucleic Acids' (p. 738) and Franklin & Gosling 'Molecular Configuration in Sodium Thymonucleate' (p. 740).
  4. 4
    nobelprize.org/prizes/medicine/1962/summary
    • Watson, Crick and Wilkins shared the 1962 Nobel Prize in Physiology or Medicine; Franklin died in 1958 aged 37, and the Nobel Prize is not awarded posthumously.
  5. 5
    britannica.com/biography/Rosalind-Franklin
    • Watson, Crick and Wilkins shared the 1962 Nobel Prize in Physiology or Medicine; Franklin died in 1958 aged 37, and the Nobel Prize is not awarded posthumously.
  6. 6
    genome.gov/genetics-glossary/Base-Pair
    • DNA's two strands each have a sugar-phosphate backbone (deoxyribose sugar and phosphate) and wind into a double helix; the bases form the rungs.
    • Adenine pairs with thymine and cytosine pairs with guanine; hydrogen bonds hold the base pairs, and so the strands, together.
  7. 7
    genome.gov/about-genomics/fact-sheets/Deoxyribonucleic-Ac…
    • DNA's two strands each have a sugar-phosphate backbone (deoxyribose sugar and phosphate) and wind into a double helix; the bases form the rungs.
  8. 8
    britannica.com/science/DNA
    • The two strands of the double helix run in opposite directions (antiparallel).
    • Adenine and thymine are joined by two hydrogen bonds; cytosine and guanine by three.
  9. 9
    genome.gov/genetics-glossary/DNA-Replication
    • In replication, helicase unwinds/separates the strands by breaking hydrogen bonds; each strand acts as a template and DNA polymerase adds complementary nucleotides, so each new molecule has one old and one new strand (semi-conservative replication).
  10. 10
    britannica.com/science/DNA/Replication-of-DNA
    • In replication, helicase unwinds/separates the strands by breaking hydrogen bonds; each strand acts as a template and DNA polymerase adds complementary nucleotides, so each new molecule has one old and one new strand (semi-conservative replication).
  11. 11
    britannica.com/biography/Maurice-Wilkins
    • Maurice Wilkins was born in Pongaroa, New Zealand, on 15 December 1916; his X-ray diffraction studies of DNA were central to understanding its structure.
  12. 12
    nature.com/articles/171738a0
    • The 25 April 1953 issue of Nature carried Watson & Crick 'Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid' (p. 737), Wilkins, Stokes & Wilson 'Molecular Structure of Deoxypentose Nucleic Acids' (p. 738) and Franklin & Gosling 'Molecular Configuration in Sodium Thymonucleate' (p. 740).
  13. 13
    nature.com/articles/171740a0
    • The 25 April 1953 issue of Nature carried Watson & Crick 'Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid' (p. 737), Wilkins, Stokes & Wilson 'Molecular Structure of Deoxypentose Nucleic Acids' (p. 738) and Franklin & Gosling 'Molecular Configuration in Sodium Thymonucleate' (p. 740).
  14. 14
    commons.wikimedia.org/wiki/File:Physics_Research_Students,_Cavendish…
    • James Watson and Francis Crick were among the research students photographed at the Cavendish Laboratory, Cambridge, in June 1952.
  15. 15
    commons.wikimedia.org/wiki/File:Part_1_of_a_two_part_telegram_to_F._…
    • A telegram of 18 October 1962 from the Karolinska Institutet told Francis Crick he would share the 1962 Nobel Prize in Physiology or Medicine, one third each, with Watson and Wilkins.
  16. 16
    www1.udel.edu/chem/C465/senior/fall00/GeneticTesting/basepr.…
    • Adenine and thymine are joined by two hydrogen bonds; cytosine and guanine by three.

Image and sound credits

  • King's College, Strand, London, engraving by J. C. Carter: Wellcome Collection, CC BY 4.0, via Wikimedia Commons
  • Rosalind Franklin blue plaque: Matt Brown, CC BY 4.0, via Wikimedia Commons
  • Slide rule owned by Rosalind Franklin (Faber-Castell 44/54 Darmstadt): Science History Institute, CC BY 4.0, via Wikimedia Commons
  • View of Pongaroa, 1907: Auckland Libraries Heritage Collections, no known copyright restrictions
  • Maurice Wilkins memorial, Pongaroa (2009): LawrieM, public domain, via Wikimedia Commons
  • James Watson: National Cancer Institute, public domain
  • Francis Crick: Marc Lieberman, CC BY 2.5, via Wikimedia Commons
  • Physics research students, Cavendish Laboratory, June 1952: Wellcome Collection, CC BY 4.0
  • Replica of the 1953 DNA double helix model (Berlin): Flocci Nivis, CC BY 4.0, via Wikimedia Commons
  • Telegram to Francis Crick, 18 October 1962: Wellcome Collection, CC BY 4.0
  • DNA 'The Secret of Life' plaque, The Eagle, Cambridge: KylieTastic, CC BY 4.0, via Wikimedia Commons
  • Photograph 51 pattern, DNA ladder and replication diagrams: redrawn/drawn in code (schematic)
  • Paper textures: ambientCG (CC0) · Torn paper edges generated with AI
  • Sound effects: Freesound contributors (CC0)
  • Music: generated with ACE-Step · Voice: Gemini TTS

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