One of the purines(adenine and guanine)or pyrimidines(cystosine, thymine, and uracil)that occurs attached to the sugar component of DNA or RNA.

• 碱基嘌呤（腺嘌呤和鸟嘌呤）或嘧啶（胸嘧啶、胸腺嘧啶和尿嘧啶）的一种，附着在脱氧核糖核酸或核糖核酸的糖分子上