Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Page 5 of 13
Discovering the Structure of DNA

With the help of the research of others, Watson and Crick figured out how the bases of DNA joined each other to form a double helix.

James D. Watson and Francis Crick based their molecular model of the DNA double helix on data that had been collected by many researchers in other laboratories. They were the first to put together all of the scattered fragments of information that were required to produce a successful molecular model of DNA. Much of the data that was used by Crick and Watson came from Linus Pauling at the California Institute of Technology and from work by Maurice Wilkins, Rosalind Franklin, A.R. Stokes, and H.R. Wilson at King's College in London.

In particular, X-ray diffraction patterns obtained by Rosalind Franklin, as shown in the above figure, revealed great symmetry and consistency in the structure of DNA and gave important clues about its dimensions. Watson and Crick had access to some of Franklin's unpublished data from a source that she was not aware of. They also saw, without her permission, the B-DNA X-ray diffraction pattern she obtained and the progress report she submitted to the Medical Research Council. Franklin's X-ray diffraction pictures and her explanation of them were the final pieces of the puzzle for Watson and Crick, and they raced to publish the structure before anyone else.
In a brief article called "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid," published in the scientific journal Nature in 1953, Watson and Crick first described the double helix structure of DNA. This publication had a major impact on both its authors and the future of biological studies, particularly in the field of genetics.

The Watson and Crick model showed DNA as a double helix, with the sugar and phosphate parts of nucleotides forming the two strands of the helix and the nucleotide bases pointing into the helix and stacking on top of each other. The specific pairing of nucleotide subunits was a key feature of the Watson and Crick model of DNA, and was an outgrowth of Erwin Chargaff's work, which he had shared with them. In DNA, the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. The A:T and C:G pairs are structurally similar. In particular, the length of each base pair is the same and they fit equally between the two phosphate backbones. The base pairs are held together by hydrogen bonds, as shown in (Figure 1), a type of chemical attraction that is easy to break and easy to reform. After realizing the structural similarity of the A:T and C:G pairs, Watson and Crick soon produced their double helix model of DNA. With the hydrogen bonds at the core of the helix providing a way to unzip the two complementary strands for easy replication, this became the last key requirement for a likely model of the genetic molecule.

Watson and Crick, along with Wilkins, received a Nobel Prize for the double helix model in 1962. Rosalind Franklin died prior to the Nobel award and was therefore ineligible.