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. In RNA the adenine actually pairs with the uracil because we no longer have the thionine. So in RNA molecules. Our adenine pairs with your cell while the guanine pairs with our cytosine.