and replace just one hydrogen with chlorine? You get 6-chlorouracil. Uracil itself is one of the basic pyrimidine bases used in RNA. But the chlorine changes the molecule's role. To a synthetic chemist, that Cl atom is a replaceable connection point.
A nucleophile such as an amine or hydrazine can push the chlorine out and install something new at exactly that position. Now the molecule can keep growing. Hydrazine, for example, can turn 6-chlorouracil into a hydrazino derivative.
Global Chemical Database