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. So this is the major difference between the thymine and our uracil nitrogenous base. So in DNA molecules we have the thymine but in RNA molecules the thymine is replaced with our uracil. Now recall that in DNA the thymine combines or pairs with the adenine.