USA Biology Olympiad (USABO) Practice Exam

Why are isolated RNA molecules generally less stable than DNA at physiological pH?

1. RNA contains deoxyribose.

2. RNA contains ribose.

3. RNA is single-stranded.

4. RNA has a shorter lifespan in cells.

The correct answer is: RNA contains ribose.

Isolated RNA molecules are generally less stable than DNA at physiological pH primarily due to the presence of ribose in their structure. Ribose, which is a sugar with a hydroxyl (-OH) group attached to the 2' carbon, makes RNA more susceptible to hydrolysis. This hydroxyl group can participate in reactions that lead to the cleavage of the RNA backbone, particularly in the presence of alkaline conditions. On the other hand, DNA contains deoxyribose, where the 2' carbon lacks this hydroxyl group, making the DNA backbone more stable and less prone to hydrolytic degradation. The structure of RNA, usually existing as a single strand, further exposes it to potential enzymatic degradation. While the lifespan of RNA in cells can be affected by various factors such as RNA processing and degradation mechanisms, the inherent chemical properties of ribose contribute significantly to the overall instability of RNA compared to DNA under physiological conditions.