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According to Watson and Crick DNA molecule consists of two such polynucleotide chains wrapped helically around each other, with the sugar- phosphate chain on the outside (forming ribbon like back bone of double helix) and purines and pyrimidines on the inside of the helix (projecting between two sugar-phosphate backbones as transverse bars). The two polynucleotide strands are held together by hydrogen bonds between specific pairs of purines and pyrimidines.
The hydrogen bonds between purines and pyrimidines are such that adenine can bond only to thymine by two hydrogen bonds, and guanine can bond only to cytosine by three hydrogen bonds and no other alternative is possible between them. The specificity of the kind of hydrogen bonds that can be formed assures that for every adenine in one chain there will be thymine in the other.
For every guanine in first chain there will be a cytosine in the other and so on. Thus, the two chains are complementary to each other; that is sequence of necleotides in one chain dictates the sequence of nucleotides in the other. The two strands run antiparallely – that is, have opposite directions. One strand has phosphodiester linkage in 3′ – 5′ direction, while other strand has phosphodiester linkage in just reverse or 5′ – 3′ direction.
Further, both polynucleotides strands remain separated by 20 A distance. The coiling of double helix is right handed and a complete turn occurs every 34 A. Since each nucleotide occupies 3.4 A distance along the length of a polynucleotide strand, ten mononucleotides occur per complete turn.
The most crucial outcome of Watson and Crick’s double helix model of DNA was the implicit suggestion for a mechanism by which the genetic material (DNA) can be copied and transmitted to progeny. This is known as DNA replication.