The answer is; Uracil, Thiamine
The two different structurally, with thymine having a methyl group in its ring structure which uracil lacks. DNA does not have uracil because Cytosine can be tautamerized and deaminate to produce uracil. Uracil is recognized by repair enzyme of DNA, nicked and replaced by cytosine. If uracil were a component of DNA, the repair systems would not be able to distinguish original uracil from uracil originated by deamination of cytosine. Thymine is more stable than uracil, a requirement for DNA which stores critical cell information.