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Why does each base in DNA have a complementary base? Complementary bases are a key feature of DNA structure. Adenine and thymine are complementary to one another as are guanine and cytosine. There are two important reasons why DNA is composed of complementary bases. The first is that only with complementary bases can the two strands of DNA have complementary sequences. Without the complementary sequences DNA replication as we know it would be impossible. The second reason is that the pairs of bases are complementary because their chemical structure allows for strong hydrogen bonding and they fit together well. The strong association between complementary bases makes DNA a stable molecule and aids in the detection of errors in DNA synthesis.