Double Helix of DNA DNA is a long molecule formed by two long polynucleotide strands held together by hydrogen bonds. These bonds occur between complementary pairs of nitrogen bases. The pairing of nitrogen bases occurs according to a predictable pattern: Adenine pairs with Thymine by two hydrogen bonds (A = T) and Cytosine with Guanine by three hydrogen bonds (C = G). This complementarity is known as the base-pairing rule. One end of the strand is called its 5’ end. The last deoxyribonucleotide at that end has the 5’ - C of its deoxyribose free. The other end of the strand is called its 3’ end because the 3’ - C of deoxyribose of the last nucleotide at that end is also free. In the DNA molecule the two strands are always antiparallel to each other. That is, the 5’ end of each strand faces the 3’ end of the other. The double strand of DNA is coiled upon itself forming a double helix, like a spiral staircase with the sugar-phosphate units along the railing and the hydrogen-bonded base pairs as the steps. This is known as the Watson and Crick model.
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