Final answer:
Thymine, found in DNA, has an extra methyl group compared to uracil, which is found in RNA. The presence of ribose sugar in RNA and deoxyribose in DNA also differentiates the molecules in which they are found.
Step-by-step explanation:
Thymine and uracil differ biochemically in their structure; thymine has a methyl group that uracil lacks. Both compounds are nitrogenous bases, where thymine is found exclusively in the structure of DNA and uracil replaces thymine in RNA molecules. Thymine pairs with adenine using two hydrogen bonds, as does uracil in RNA. Additionally, thymine pairs within the double-strand DNA helix, while uracil is found in the single-stranded RNA structure.
The sugar present in the nucleotides also differs: DNA nucleotides contain the sugar deoxyribose, while RNA contains ribose sugar. Thus, the nucleotides in RNA consist of ribose, uracil, and a phosphate group, whereas DNA nucleotides are composed of deoxyribose, thymine (or adenine, cytosine, guanine), and a phosphate group.