How are nitrogenous bases connected to sugar in a nucleic acid?

The correct answer is Option (3) → N-glycosidic bond

A nucleotide has three parts: a nitrogenous base, a pentose sugar and a phosphate group. Different bonds join these parts. The base joins the sugar to form a nucleoside. A phosphate then joins the nucleoside to form a nucleotide, and nucleotides join one another to form a chain.

Option (1) Hydrogen bond — Incorrect. Hydrogen bonds hold the bases of the two strands together in double-stranded DNA (A with T, G with C). They do not join a base to its own sugar.

Option (2) Phosphodiester bond — Incorrect. A 3′-5′ phosphodiester linkage joins one nucleotide to the next, forming the sugar-phosphate backbone of the chain. It does not link the base to the sugar.

Option (3) N-glycosidic bond — Correct. The nitrogenous base is linked to the OH of the 1′ carbon of the pentose sugar through an N-glycosidic linkage. The product is a nucleoside such as adenosine or deoxyadenosine.

Option (4) O-glycosidic bond — Incorrect. The base is joined to the sugar through one of its nitrogen atoms, which is why the bond is called N-glycosidic. The book does not describe an O-glycosidic bond in nucleic acids.

NCERT Extract:
Structure of Polynucleotide Chain – “… A nitrogenous base is linked to the OH of 1' C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine. When a phosphate group is linked to OH of 5' C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed. Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide.”