Genetics and Molecular Biology Practice Exam 2026 – Comprehensive Study Resource

The thing being tested is the difference in nitrogenous bases between DNA and RNA. DNA uses thymine, while RNA uses uracil in place of thymine. Because thymine is present in DNA but not in RNA, thymine is the base found in DNA but not in RNA. Uracil serves the role that thymine would in RNA, and thymine’s extra methyl group distinguishes it chemically as a deoxyribose-based base. So the correct base is thymine.