In the 1950s, a young American biologist, James Watson, teamed up with British graduate student Francis Crick at Cambridge University in England to try to determine the structure of DNA. By 1953, they had put together a model for the structure of DNA as shown in Figure 10-4. They proposed that DNA is made of two chains that wrap around each other in the shape of a double helix, a shape similar to a winding spiral staircase. Their final model was correct and was remarkable because it explained how DNA could replicate.
Watson and Crick relied on other scientists’ work to develop their DNA model. Part of that work was X-ray diffraction photographs of DNA crystals. The photographs and crystals were produced by researchers Rosalind Franklin and Maurice Wilkins, at King’s College in London.
In 1962, Watson, Crick, and Wilkins received the Nobel Prize in Medicine for their work on DNA. Rosalind Franklin died in 1958 and so could not be named in the award. However, an important genetics institute in Cambridge now bears her name, and her contribution is recognized around the world.
DNA is a nucleic acid made of two long chains (also called strands) of repeating subunits called nucleotides. Each nucleotide consists of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The five-carbon sugar in a DNA nucleotide is called deoxyribose. The phosphate group consists of a phosphorus (P) atom bonded to four oxygen (O) atoms. The nitrogenous base contains nitrogen (N) atoms and carbon (C) atoms and is a base (accepts hydrogen ions).
The DNA double helix is similar to a spiral staircase. The alternating sugar and phosphate molecules form the side “handrails” of the staircase. Nucleotides along each strand are connected by covalent bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide. Each full turn of the DNA helix has 10 nucleotide pairs.