In $\text{1953}$, J.D. Watson (an American biologist) and F.H.C. Crick (a British physicist) proposed a three-dimensional model of physiological DNA (i.e., B-DNA) based on X-ray diffraction data of DNA obtained by Franklin and Wilkins. For this epoch-making discovery, Watson, Crick and Wilkins got Nobel Prize in medicine in 1962. According to them, the diameter of DNA is 20 nm or 20 Angstroms. Adjacent bases are separated 0.34 nm or by 3.4 Angstroms along the axis. The length of a complete turn of helix is 3.4 nm or 34 Angstroms, i.e., there are 10 bp per turn/pitch. Thus, 15 pitches will have 150 bp, i.e., 300 nucleotides.