Nucleotides are joined together by

The correct answer is Option (1) → Phosphodiester linkage

A nucleotide has three parts: a nitrogenous base attached to the 1′ position of a pentose sugar (this unit is a nucleoside), and a phosphoric acid unit attached at the 5′ position of the sugar. In a nucleic acid, the phosphate of one nucleotide forms an ester bond with the 3′-OH of the sugar of the next nucleotide. Each phosphate is thus esterified twice, to the 5′ carbon of one sugar and the 3′ carbon of another, which is why the link is called a phosphodiester linkage:

    sugar(3′)–O–P(O)(OH)–O–(5′)sugar

A chain of such links forms the sugar–phosphate backbone of DNA and RNA.

Why the other options are incorrect

• Option (2) Glycosidic linkage — Incorrect. The oxide link that joins monosaccharide units in disaccharides and polysaccharides (e.g. sucrose, starch).

• Option (3) Peptide linkage — Incorrect. The amide link (–CO–NH–) that joins amino acids in proteins.

• Option (4) Hydrogen bond — Incorrect. Holds the two strands of DNA together through specific base pairs (A with T, G with C); it does not join nucleotides within a strand.

NCERT (2026-27 edition):
Structure of Nucleic Acids – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: