DNA consists of two strands of polynucleotides coiled around each other in the form of a double helix.The base of one strand of DNA are paired with bases on other strand by means of hydrogen bonding.This hydrogen bonding is very specific as the structures of bases permit only one mode of pairing.Adenine pair with only thymine via 2 hydogen bonds and guanine pairs with cytosine through 3 hydrogen bonds.The two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically determines that of the other.This specific base pairing is the most important principle of the structure od DNA.
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In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases. Cytosine forms hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result, the two strands are complementary to each other.