DNA STRUCTURE
Read the following passage carefully and answer the given questions.
DNA is a long polymer of deoxyribonucleotides. A nucleotide has three components – a nitrogenous base, a pentose sugar, and a phosphate group. There are two types of nitrogenous bases – Purines and Pyrimidines. DNA is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside. The bases in two strands are paired through hydrogen bond forming base pairs. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix. The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability of the helical structure. The two polynucleotide chains are coiled in a right-handed fashion. In eukaryotes, the DNA organisation is much more complex. There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains.
To form a nucleoside, the nitrogenous base is linked to the:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → OH of 1′C pentose sugar
A nucleoside is a nitrogenous base joined to a pentose sugar. The base is linked to the OH of the 1′ carbon of the sugar by an N-glycosidic linkage. Examples are adenosine and guanosine. When a phosphate group is then joined to the OH of the 5′ carbon, a nucleotide is formed.
(1) OH of 3′C pentose sugar — Incorrect. The 3′ carbon is where the next nucleotide is joined, by a 3′-5′ phosphodiester linkage.
(2) OH of 1′C pentose sugar — Correct. This is where the base is attached.
(3) OH of 4′C pentose sugar — Incorrect. The base is not attached at this carbon.
(4) OH of 2′C pentose sugar — Incorrect. The 2′ carbon carries an extra OH in the ribose of RNA. It is not where the base joins.
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: A similar question was also asked in CUET (UG) 2026 on 06-06-2026 (Slot 1) and on 26-05-2026 (Slot 1).
NCERT Extract:
Structure of Polynucleotide Chain – …
