there are 4 bases Adenine (A) Cytosine (C) Thymine (T) Guanine (G) Adenine only pairs up with Thymine and vice verca. Cytosine only pairs up with guanine and vice verca. A way that my bio teacher...
In DNA the base pairs are Adenine with Thymine and Guanine with Cytosine. In RNA Thymine is replaced by Uracil so the base pairs are Adenine with Uracil and Guanine with Cytosine.