Two most important criteria of this structure are base corn plensentarity and antipnrnlklisrn

Two most important criteria of this structure are base corn plensentarity and antipnrnlklisrn that explain the biological utility of DNA. Two chains of the helix are antiparallel to each other, i.e., they have opposite polarity. This helps in positioning the sugar phosphate backbone towards

outside of the helix, while the bases remain inside. The nitrogenous bases in one chain are complementary to that of the other chain being joined by hydrogen bonding. Adeninc in on chain always pairs with thymine with two hydrogen bonds, while guanine pairs with cytosine with three hydrogen bonds. In this way, if the sequence of bases of one chain is known, the composition of the other chain can definitely be predicted. Exploiting these features, DNA replicates itself, disseminates information by synthesizing RNA and allows mutational and recombinational changes to account for genetic variability. The double helix has a constant diameter of 20 A, which is sufficient to fit one purine-pyrimidine hydrogen bonding combination. Each

nucleotide of the chain shos’s a restricted rotation of 36° due to phosphodiester bridge formation. A complete revolution of 360° involves 10 base pairs. The distance that a double helix covers in one complete turn is 34 A, distance between each base pair being 3.4 A. The right handed B-DNA is the most abundant DNA found in nature, although slight variations exist from the proposed Watson-Crick structure. The B-DNA found in nature has about 10.5 base pairs per turn of helix. Alternate forms of DNA can be

found depending on the relative humidity and salt concentration of the environment as well as the composition of the nucleotides. The A-DNA, induced under low relative humidity is a right handed fatty helix with diameter about 26 A, ii base pairs per turn of helix and a helical twist of 33° per base rise and a helix

pitch (distance per complete turn) of 28 A.