DNA : The Structure and Function

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A deoxyribonucleic acid (DNA) molecule consists of two long polynucleotide
chains composed of four types of nucleotide subunits. Each of these chains is
known as a DNA chain, or a DNA strand. The chains run antiparallel to each other,
and hydrogen bonds between the base portions of the nucleotides hold the two
chains together

DOUBLE HELIX

The three-dimensional structure of DNA—the DNA double helix—arises from
the chemical and structural features of its two polynucleotide chains. Because
these two chains are held together by hydrogen-bonding between the bases on
the different strands, all the bases are on the inside of the double helix, and the
sugar-phosphate backbones are on the outside In each case, a
bulkier two-ring base is paired with a single-
ring base (a pyrimidine): A always pairs with T, and G with C