Step 1: Understanding the Question:
The question asks to identify the structural difference in the pentose sugar backbone of the two primary nucleic acids, DNA and RNA.
Step 2: Detailed Explanation:
Both DNA and RNA are nucleic acids built from nucleotides, which consist of a phosphate group, a nitrogenous base, and a 5-carbon (pentose) sugar. However, the exact type of sugar differs:
RNA (Ribonucleic Acid): The sugar present is Ribose. Ribose has a hydroxyl group ($-OH$) attached to its $2'$ carbon atom.
DNA (Deoxyribonucleic Acid): The sugar present is Deoxyribose. As the prefix "deoxy-" implies, this sugar is missing an oxygen atom at the $2'$ carbon position (it has an $-H$ instead of an $-OH$).
This single missing oxygen atom is what makes DNA much more chemically stable than RNA, making it the ideal molecule for long-term genetic storage.
Step 3: Final Answer:
The primary difference is that DNA contains deoxyribose while RNA contains ribose.