Salient Features of DNA Double-Helix\tIt consists of two polynucleotide chains where the sugar and phosphate group form the backbone and the nitrogenous bases project inside the helix.\tThe two polynucleotide chains have anti-parallel polarity i.e. if one strand has 5′\xa0→ 3′ polarity, the other strand has 3′\xa0→ 5′ polarity.\tThe bases on the opposite strands are connected through hydrogen bonds forming base pairs (bp). Adenine always forms two hydrogen bonds with thymine from the opposite strand and vice-versa. Guanine forms three hydrogen bonds with cytosine from the opposite strand and vice-versa. Therefore, a purine always pairs with a pyrimidine on the other strand, giving rise to a uniform distance between the two strands of the helix.\tThe two strands coil in a right-handed fashion. Each turn of the helix is 3.4nm (or 34 Angstrom units) consisting of 10 nucleotides. These nucleotides are at a distance of 0.34nm (or 3.4 Angstrom units).\tThe helix is stable because of the base pairs that stack over one another and hydrogen bonds that hold the bases together.