deoxy sugar

Meaning: In a deoxy sugar, one –OH is replaced by –H, so that carbon becomes a CH₂ instead of a CH–OH. The number in the name gives the position. 2-Deoxyribose (C₅H₁₀O₄) is ribose (C₅H₁₀O₅) minus the oxygen on C2. Other examples are L-fucose (6-deoxygalactose) and rhamnose (6-deoxymannose), which appear in glycoproteins and other glycans. In a nucleotide, the base attaches at C1′ by an N-glycosidic bond and the phosphate attaches at C5′.

On the MCAT: A passage compares DNA and RNA and asks why DNA suits long-term storage of genetic information. In RNA, the 2′-OH can act as a nucleophile on the neighboring phosphorus and cleave the phosphodiester backbone. DNA has no 2′-OH, so it is more resistant to hydrolysis. You may also be asked to identify deoxyribose from a structure by counting oxygens, or to recall that ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides.

Common confusion: Students sometimes think deoxyribose lacks the 3′-OH. It does not. The 3′-OH is present and is what the incoming nucleotide's phosphate reacts with during chain elongation. A dideoxy sugar, lacking both 2′-OH and 3′-OH, stops chain growth, which is the basis of Sanger sequencing. "Deoxy" describes the sugar only. The thymine versus uracil difference between DNA and RNA is a separate point.