Watson and Crick shared the Nobel Prize with Wilkins for showing the molecular structure of DNA. Wilkins used X-ray crystallography and showed that two antiparallel polynucleotides chains of DNA are coiled around the same axis while bases are present at a right angle to the axis. Since, one complete turn of the double helix is 3.4 nm long and each of which has 10 base pairs, the distance between two consecutive base pairs 0.34 nm. Watson and Crick postulated the 3-D model of DNA using all available data. According to their model, the DNA molecule is made up of two helical DNA chains that are wound around the same axis. The hydrophilic backbones of alternating sugar and phosphate groups are present on the outside of the double helix facing the surrounding water. The nitrogenous bases, purine and pyrimidine, are stacked inside the double helix. Each nucleotide base of one strand is paired in the same plane with a base of the other strand.