减基对:A=T,G=C NHe H 嘌呤 H一c Purines C C N N NH H H H 鸟嘌呤 Guanine,G 腺嘌呤 Adenine,A NHa 2 H H3c H C 嘧啶 H N Pyrimidines H H 胞嘧啶 Cytosine,C胸腺嘧啶 Thymine,T碱基对：A=T, G≡C 嘌呤 Purines 鸟嘌呤Guanine,G 腺嘌呤Adenine,A 嘧啶 Pyrimidines 胞嘧啶Cytosine,C 胸腺嘧啶Thymine,T