ATCG是主要碱基.
修饰碱基：除主要碱基外,核酸中也有一些含量很少的稀有碱基.稀有碱基的结构多种多样,多半是主要碱基的衍生物,所以也叫修饰碱基.
tRNA往往含有较多的稀有碱基,有的tRNA含有的稀有碱基达到10％.
Primary bases are incorporated into the growing chain during RNA and/or DNA synthesis.Apart from adenine (A),cytosine (C),guanine (G),thymine (T) and uracil (U),DNA and RNA also contain bases that have been modified after the nucleic acid chain has been formed.In DNA,the most common modified base is 5-methylcytidine (m5C).In RNA,there are many modified bases,including pseudouridine (Ψ),dihydrouridine (D),inosine (I),ribothymidine (rT) and 7-methylguanosine (m7G).
英文来自WIKIPEDIA 和中文不完全对英,但也差不多.你高中应该看的懂的