Genetics and Molecular Biology Practice Exam 2026 - Free Molecular Biology Practice Questions and Study Guide

Uracil is the nitrogen base used in RNA in place of thymine. In RNA, uracil pairs with adenine during base pairing, whereas in DNA thymine takes that spot and pairs with adenine. Thymine is essentially a methylated version of uracil and is specific to DNA, contributing to greater chemical stability and helping repair systems distinguish true mutations (like deamination events) from normal bases.

The other bases listed—adenine and cytosine—are found in both RNA and DNA, so they don’t fit the idea of being present only in RNA. Thymine is found in DNA, not RNA, so it also doesn’t fit.