碱基对是形成核酸DNA、RNA单体以及编码遗传信息的化学结构。组成碱基对的碱基包括腺嘌呤（A）、胸腺嘧啶（T）、鸟嘌呤（G）、胞嘧啶（C）、尿嘧啶（U）。严格地说，碱基对是一对相互匹配的碱基（即A：T， G：C，A：U相互作用）被氢键连接起来。然而，它常被用来衡量DNA和RNA的长度（尽管RNA是单链）。它还与核苷酸互换使用，尽管后者是由一个五碳糖、磷酸和一个碱基组成。

碱基对通常简写做bp（base pair）；千碱基对 为kbp，或简写作kb（对于双链核酸。对於单链核酸，kb指千碱基）；兆碱基即百万对碱基简写作Mbp。

A base pair (bp) is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix, and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson-Crick base pairs (guanine-cytosine and adenine-thymine) allow the DNA helix to maintain a regular helical structure that is subtly dependent on its nucleotide sequence. The complementary nature of this based-paired structure provides a backup copy of all genetic information encoded within double-stranded DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA, and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base pairing patterns that identify particular regulatory regions of genes.