base pair
(bays payr)
Molecules called nucleotides, on opposite strands of the DNA double helix, that form chemical bonds with one another. These chemical bonds act like rungs in a ladder and help hold the two strands of DNA together. There are four nucleotides, or bases, in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). These bases form specific pairs (A with T, and G with C).
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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)].](https://nci-media.cancer.gov/pdq/media/images/686274-571.jpg)
Enlarge this image in new windowDNA 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 .