In DNA, the two purine bases (adenine and guanine) always bond with the pyrimidine bases (thymine and cytosine), and the pairing is quite specific: adenine with thymine and guanine with cytosine.

• 嘌呤基（腺嘌呤，鸟嘌呤）总与嘧啶基（胞嘧啶，胸腺嘧啶）配对，并且配对有专一性：腺嘌呤对胸腺嘧啶，鸟嘌呤对胞嘧啶。