The Discovery of the Double Helix


Eighteenth Annual Darwin College Lecture Series 2003

Lecture 2   :   24 January 2003

Professor Sir Aaron Klug
MRC Laboratory of Molecular Biology, Cambridge


The structure of the DNA double helix, with its complementary base pairing, is one of the greatest discoveries in biology in the 20th Century. It was also most dramatic, since, quite unexpectedly, the structure itself pointed to the way in which a DNA molecule might replicate itself, and hence revealed the "secret of life". The structure was solved in the Cavendish Laboratory, Cambridge by Francis Crick and James Watson, using X-ray diffraction data from fibres of DNA obtained by Rosalind Franklin at King's College, London. The lecture aims to tell the story of the origin of the research on DNA, the early experiments by Maurice Wilkins at King's College, the sorting out of the two forms of DNA by Franklin, the wrong paths taken, the interplay of old rivalries, and the final model-building by Watson and Crick to give the three dimensional structure.

The initial, often hesitant, reception of the proposed structure, and its final confirmation by biochemistry and X-ray crystallography, will be described.

Finally, a very brief summary will be given of the results of 50 years of research on the complex "molecular machines" which carry out the two main functions of DNA, replication of the molecule, and transcription (reading) of the sequence of the DNA to produce the RNA coding for the protein product of a gene.


Aaron Klug was educated at the Universities of Witwatersrand, Cape Town and Cambridge. He began as a medical student, transferred to science, and his PhD at the Cavendish Laboratory was in Physics. He joined the MRC Laboratory of Molecular Biology in Cambridge in 1962, was the Director of the Laboratory from 1986 to 1996, and now continues as a member of staff, leading a research group on gene expression.

He was a colleague of Rosalind Franklin at Birkbeck College in the 1950s soon after the time when her X-ray diffraction of DNA provided key information which allowed Watson and Crick to propose the double helical structure.

His own work has been on the interactions of proteins and nucleic acids and on the elucidation of the structures of large biological molecules and assemblies, including simple viruses and chromatin, by X-ray diffraction and electron microscopy and on the development of new methods for their study. The principle of his method of 3-D image reconstruction in electron microscopy from a series of 2-D tilted images later formed the basis of X-ray CT scanner. In 1982 he was awarded the Nobel Prize in Chemistry. His current research is on the structure of DNA and RNA binding proteins which regulate gene expression and in particular on the interaction with DNA and RNA of the zinc finger family of transcription factors which he discovered.

He was President of the Royal Society (1995-2000), is a member of the Order of Merit, a Foreign Associate of the US National Academy of Sciences, and of the French Academy of Sciences, and has received many honorary degrees. He is an Honorary Fellow of Peterhouse and of Trinity College, Cambridge.

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