Step 1: Pentose sugar in DNA.
DNA contains 2-deoxyribose, a five-carbon sugar whose 2' carbon carries only $-H$ (no hydroxyl group).
Step 2: Pentose sugar in RNA.
RNA contains ribose, whose 2' carbon bears a hydroxyl group ($-OH$), making ribose one oxygen atom richer than deoxyribose.
Step 3: The structural difference and its significance.
DNA uses 2-deoxyribose while RNA uses ribose. The presence of the 2'-OH in ribose makes RNA chemically more reactive and less stable than DNA, which is why DNA is better suited for long-term genetic storage.