尿嘧啶uracil 胞嘧啶cytosine 胸腺嘧啶thymine NH N H O O N N H NH2 O NH N H O O N NH 1 3 2 4 5 6尿嘧啶uracil 胞嘧啶cytosine 胸腺嘧啶thymine NH N H O O N N H NH2 O NH N H O O N NH 1 3 2 4 5 6