Structure : Using the methods of X- Ray diffraction, Rosalind Franklin and Maurice Wilkins showed that DNA produces a characteristic X- Ray diffraction pattern from which James Watson and Francis Crick deduced the double helical model of DNA structure showing two polynucleotide chains running in opposite directions and coiled around each other. Erwin Chargoff's findings published in 1949 was of great help in solving base-pairing mystery. He showed that even though different organisms have different amounts of DNA, percentage of adenine always equals thymine and percentage of guanine equals cytosine. This was possible only when adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C) in the double helix. Within the DNA double helix, adenine forms two hydrogen bonds with thymine on the opposite strand and guanine forms three hydrogen bonds with cytosine on the opposite strand. For discovery of DNA structure, Francis Crick, James Watson and Maurice Wilkins awarded Nobel Prize in Physiology and Medicine (1962).