Determining the structure for DNA was one of the outstanding science achievements of the 20th century.
It started in the late 1940's, when scientists were aware that DNA was most likely the molecule of life, even though many were sceptical since it was so "simple." They also knew that DNA included different amounts of the four bases: adenine, thymine, guanine and cytosine (usually abbreviated A, T, G and C), but nobody knew what the molecule might look like.
Some years later the base-pairing system had been partly solved by the biochemist Erwin Chargaff. In 1949 he showed that even though different organisms have different amounts of DNA, the amount of adenine always equals the amount of thymine. The same goes for the pair guanine and cytosine.
Maurice Wilkins and Rosalind Franklin were peers at King’s College. Franklin had discovered that DNA could crystallize into two different forms, an A form and a B form. The technique with which Wilkins and Franklin set out to do is called X-ray crystallography. With this technique a crystal is exposed to x-rays in order to produce a diffraction pattern. If the crystal is pure enough and the diffraction pattern is developed very carefully, it is possible to reconstruct the positions of the atoms in the molecules that contain the basic unit of the crystal called the unit cell. By the early 1950s, scientists were just learning how to do this for biological molecules as complex as DNA.
After discovering the existence of the A and B forms of DNA, Rosalind Franklin went on to achieve excellent X-ray diffraction patterns of crystalline B-form DNA and, using a combination of crystallographic theory and chemical reasoning, discovered important basic facts about its structure. She gave quantitative details about the shape and size of the double helix. The all- important missing piece of the puzzle that she could not discover from her data was how the bases paired on the inside of the