The following pentose sugar is obtained by complete hydrolysis of DNA, along with phosphoric acid and nitrogeneous base:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → β-D-2-deoxyribose
Complete hydrolysis of a nucleic acid gives a pentose sugar, phosphoric acid and nitrogenous bases. In DNA the sugar is β-D-2-deoxyribose; in RNA it is β-D-ribose. “2-Deoxy” means that C-2 carries two hydrogens and no –OH, while the –OH at C-3 remains (it forms the phosphodiester link in the chain).
In structure (2) the furanose ring has \( \ce{CH2OH} \) at C-5, the anomeric –OH at C-1, an –OH at C-3 and only H at C-2, which is β-D-2-deoxyribose.
Why the other options are incorrect
Option (1) β-D-ribose — Incorrect. It has –OH groups at both C-2 and C-3; this is the sugar of RNA.
Option (3) –OH at C-2 drawn above the ring and –OH at C-3 below — Incorrect. This is the C-2 epimer of ribose (β-D-arabinose); it still carries an –OH at C-2, so it is not the deoxy sugar of DNA.
Option (4) no –OH at C-2 or C-3 — Incorrect. This is a 2,3-dideoxy sugar; without the 3-OH the sugar cannot form the phosphodiester bond of the DNA chain.
NCERT (2026-27 edition):
Chemical Composition of Nucleic Acids
