CH CH 组 N N HaN Guanine 成 ur 核 酸 HN CH CH CH 的 CH CH H H 碱 Thymine Cytosine IDNA TRNA) 基 Pyrimidine Figure 12-2 The major purine and pyrimidine bases of nucleic acids. Some of the common names of these bases reflect the circumstances of their dis- covery Guanine, for example, was first isolated from guano(bird m ) and thymine isolated from thymus tissue.组成核酸的碱基