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NCI Dictionary of Genetics Terms

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base pair

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Two nitrogen-containing bases pair together between double-stranded DNA; only specific combinations of these bases (e.g., adenine with thymine; guanine with cytosine) are possible, a fact which facilitates accurate DNA replication; when quantified (e.g., 8 base pairs, or bp), this term refers to the actual number of base pairs in a sequence of nucleotides.

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DNA base pair; drawing shows double-stranded DNA with bonds between two pairs of nitrogen-containing bases [adenine (A) and thymine (T), and cytosine (C) and guanine (G)].
DNA base pair. Under normal circumstances, the nitrogen-containing bases adenine (A) and thymine (T) pair together, and cytosine (C) and guanine (G) pair together. The binding of these base pairs forms the structure of DNA.