  The four bases that make up RNA are:   * Adenine (A)  * Cytosine (C)  * Guanine (G)  * Uracil (U).denine and guanine are complementary, as are thymine and cytosine. RNA is practically the same, … except that thymine is replaced with uracil (U) - meaning that when RNA replicates DNA, any thymine needed is replaced with uracil. Uracil is still compatible with cytosine, though.