The structure of which of the following sugars is a part of DNA molecule?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2-deoxyribose (structure 4)
A nucleic acid is a chain of nucleotides, each made of a pentose sugar, a nitrogenous base and a phosphate group. The two nucleic acids differ in their sugar:
RNA: β-D-ribose, with –OH groups on both C-2 and C-3.
DNA: β-D-2-deoxyribose, in which the –OH on C-2 is replaced by H (“deoxy” = one oxygen less).
In the five-membered ring drawn, C-1 is the carbon next to the ring oxygen that carries –OH, C-2 and C-3 are the two lower carbons, and C-4 carries the –\( \ce{CH2OH} \) group (C-5). In structure 4, C-2 carries two hydrogens and no –OH, C-3 carries –OH, and C-4 carries –\( \ce{CH2OH} \). This is 2-deoxyribose, the sugar of DNA.
Why the other options are incorrect
Option (1) structure 1 — Incorrect. It has –OH on both C-2 and C-3; this is ribose, the sugar of RNA.
Option (2) structure 2 — Incorrect. It still has an –OH on C-2, so it is not a 2-deoxy sugar.
Option (3) structure 3 — Incorrect. C-4 carries a –CHO group instead of –\( \ce{CH2OH} \), and C-2 has an –OH; it is not the sugar of either nucleic acid.
NCERT (2026-27 edition):
Chemical Composition of Nucleic Acids
