And the thymine and the cytosine are the two types of pyrimidines that exist in DNA. Now RNA nucleotides also contain adenine guanine as well as cytosine. However, thiamine is not found in RNA molecules. In RNA molecules the thymine is replaced with the uracil nitrogenous base and the major difference between the thymine and the uracil is the composition of atoms. So notice this carbon contains the methyl group on thymine but this carbon does not. And this carbon contains the nitrogen the amine group while this carbon contains our carbon oxygen double bond.