In the DNA molecule there is the pairing of a purine nitrogen base (adenine and guanine) with a pyrimidine (cytosine and thymine) and this complementary pairing is not random: Adenine always pairs with Thymine through two hydrogen bridges, and Cytosine always partners with Guanina for three hydrogen bridges. Thus, the amount of adenine is always equal to that of thymine and the amount of cytosine is the same as that of guanine. However, the amount of A + T is not necessarily equal to C + G.
With this, we can say that the complementary base to thymine is adenine.
