The base thymine appears in DNA but in RNA it is replaced by:

1. DNA carries the bases adenine, thymine, guanine and cytosine. 2. RNA carries adenine, guanine and cytosine as well, but not thymine. 3. In RNA the base T is absent and U is present instead. 4. So thymine is replaced by uracil. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_