A nucleotide is hydrolysed completely. The products are thymine, a pentose sugar and phosphoric acid. Which of the following is correct about this nucleotide?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It comes from DNA, and its sugar is β-D-2-deoxyribose.
A nucleotide is a base attached at the 1′ position of a pentose sugar, with phosphoric acid linked at the 5′ position. Complete hydrolysis gives back all three parts:
Nucleotide + Water → Thymine + 2-Deoxyribose + Phosphoric acid
The base decides the source. Thymine is found only in DNA; in RNA its place is taken by uracil. The sugar of DNA is β-D-2-deoxyribose, so the pentose obtained is 2-deoxyribose.
Why the other options are incorrect
Option (1) It comes from RNA, and its sugar is β-D-ribose. — Incorrect. An RNA nucleotide would give uracil, never thymine.
Option (2) It comes from DNA, and its sugar is β-D-ribose. — Incorrect. β-D-Ribose is the sugar of RNA; DNA contains 2-deoxyribose.
NCERT (2026-27 edition):
10.5.1 Chemical Composition of Nucleic Acids – “Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose.”
“DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).”