Molecular Basis of Inheritance
Read the following passage carefully and answer the given questions.
A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA and deoxyribose in case of DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs). The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA). It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases. He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of 43 (4 × 4 × 4) would generate 64 codons; generating many more codons than required.
A nitrogenous base is linked to the OH of __________ through a N-glycosidic linkage to form a nucleoside.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1′C pentose sugar
A nucleoside is a nitrogenous base joined to a pentose sugar. The joining bond is an N-glycosidic linkage between the base and the OH of the 1′ carbon of the sugar. Adenosine and guanosine are examples. When a phosphate group is added to the sugar, the nucleoside becomes a nucleotide.
(1) 2′C pentose sugar — Incorrect. This carbon does not hold the base. In deoxyribose it is the carbon that has no OH, which is how DNA’s sugar differs from ribose.
(2) 5′C pentose sugar — Incorrect. The phosphate group joins the OH of the 5′ carbon of the nucleoside by a phosphoester linkage. This makes the nucleotide.
(3) 3′C pentose sugar — Incorrect. The OH on this carbon takes part in the 3′-5′ phosphodiester linkage that joins two nucleotides.
(4) 1′C pentose sugar — Correct. The base is attached here.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: The same question was also asked in CUET (UG) 2026 on 06-06-2026 (Slot 1), and a similar question in CUET (UG) 2026 on 22-05-2026 (Slot 1).
NCERT Extract:
Structure of Polynucleotide Chain – …
