Photo 51, the X-ray diffraction image whose cross pattern gave away the shape (1952)
Taken by Rosalind Franklin with her student Raymond Gosling at King's College London.
Science · Biology
DNA
1869 isolated; 1953 structure solved
Biology — molecular genetics
Deoxyribonucleic acid is the molecule that stores hereditary information in almost all living things. Two antiparallel sugar-phosphate backbones wind into a right-handed double helix, with the bases adenine, thymine, guanine and cytosine paired on the inside — A with T, G with C — so that each strand is a template for the other. Friedrich Miescher isolated it in 1869; its structure was solved in Cambridge in 1953 by James Watson and Francis Crick, crucially using X-ray diffraction data from Rosalind Franklin and Maurice Wilkins at King's College London.
How to spot them
- ▸A right-handed double helix of two antiparallel strands, about 2 nanometres wide
- ▸Four bases, pairing A–T and G–C by hydrogen bonding
- ▸Photo 51, taken by Rosalind Franklin and Raymond Gosling in 1952
- ▸Three papers in Nature on 25 April 1953
- ▸About 3.1 billion base pairs in the human version, and about two metres of it in every cell
Key facts
First isolated from pus-soaked surgical bandages in 1869 and called 'nuclein' (1869)
By the Swiss physician Friedrich Miescher, working in Tübingen.
Erwin Chargaff's rule that A always equals T and G always equals C
The base-pairing clue that Watson and Crick needed.
Watson and Crick's 1,000-word Nature paper of 25 April 1953, ending 'It has not escaped our notice…' (1953)
One of three papers in that issue; the others were by Wilkins and by Franklin and Gosling.
Avery, MacLeod and McCarty showed in 1944 that this, not protein, was Griffith's transforming principle (1944)
Their bacterial work was confirmed in 1952 by Hershey and Chase's blender experiment on phages.
The 1962 Nobel Prize in Physiology or Medicine went to three men for its structure; the woman whose images were decisive had died four years earlier (1962)
Watson, Crick and Wilkins. Rosalind Franklin died in 1958 and the prize is not awarded posthumously.
Alec Jeffreys invented fingerprinting from it in 1984 and it was first used to convict in the Colin Pitchfork case (1984)
The same technique first exonerated an innocent suspect before convicting Pitchfork in 1988.
About two metres of it is packed into every human cell nucleus, and about 3.1 billion base pairs long
Wound around histone proteins into nucleosomes and then into chromatin.
Key ideas
- Because the two strands are complementary, replication is semi-conservative: each strand templates a new partner, so every daughter molecule keeps one old strand — proved by Meselson and Stahl in 1958.
- The information is read in three-letter codons; 64 codons encode 20 amino acids plus stop signals, and the code is nearly universal across all life.
Quick-fire facts
- •Full name deoxyribonucleic acid; the 'deoxy' distinguishes it from RNA, whose sugar carries an extra oxygen.
- •Bases: adenine and guanine are purines (two rings), thymine and cytosine pyrimidines (one). A–T share two hydrogen bonds, G–C three, which is why G–C-rich DNA is harder to melt.
- •The B form makes one full turn every 3.4 nm, about 10.5 base pairs.
- •Rosalind Franklin, Francis Crick, James Watson and Lynn Margulis all have their own GoDeep entries as scientists.
- •Maurice Wilkins showed Photo 51 to Watson in 1953 without Franklin's knowledge.
- •The Human Genome Project ran from 1990 and announced completion in 2003; a truly gapless sequence was published by the Telomere-to-Telomere consortium in 2022.
- •Kary Mullis won the 1993 Nobel Prize in Chemistry for the polymerase chain reaction; Frederick Sanger won his second Chemistry Nobel in 1980 for sequencing methods.
- •Only about 1–2% of the human genome codes for protein; the rest was once dismissed as 'junk DNA'.
- •Common misconception: Watson and Crick did not discover the molecule — it had been known for over 80 years. They worked out its structure.
Drill it — 9 cards
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Sources
- Horace Freeland Judson — The Eighth Day of Creation: Makers of the Revolution in Biology
- Brenda Maddox — Rosalind Franklin: The Dark Lady of DNA
- J. D. Watson and F. H. C. Crick — Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid (Nature)