Watson and crick in 1953 came up with a model for DNA. They surjested that the S dimensional molecule is a helically coiled structure made up of double poly nucleotide (RNA is single stranded). They two polynucleotides strand are interwound on each other to form double helix. Each polynucleotide chain of double end is made up of phosphoric acid and sugar units as back bone attach to the organic bases projecting into the helix. There is specificity in these pairing such that A pair with T and G with C. the sequence of bases of one stand is complementary to the sequence of the other bases, since the two strands run in two opposite direction, they are said to be anti parallel. The organic bases are held together by weak hydrogen bonds, two between A and T, and 3 between C and G. ten pairs of bases make a complete turned which is equally to 3.4 mm. The distance between two strands is about 2mm.