DNA is composed of nucleotides, which are distinguished by the type of subunit, called a base, they contain: adenine (A), cytosine (C), guanine (G) or thymine (T).

• DNA是由四种核苷酸所组成，这四种核苷酸内分别含有不同的碱基：腺嘌呤（adenine,A）、胞嘧啶（cytosine,C）、鸟粪嘌呤（guanine,G）和胸腺嘧啶（thymine,T）。