Match List-I (Name of the Base) with List-II (Structure)

Choose the correct answer from the options given below:

(A)-(II), (B)-(IV), (C)-(I), (D)-(III)

The correct answer is Option (3) → (A)-(II), (B)-(IV), (C)-(I), (D)-(III)

Core Concept

Nitrogenous bases are identified by their ring system and functional groups:

Purines → Double ring → Adenine, Guanine

Pyrimidines → Single ring → Cytosine, Thymine

Each base has unique substituents like =O, –NH₂, and –CH₃ that help identify them.

Correct Matching

Adenine → Purine with –NH₂ → (II)

Cytosine → Pyrimidine with =O and –NH₂ → (IV)

Thymine → Pyrimidine with two =O and CH₃ → (I)

Guanine → Purine with =O and –NH₂ → (III)

Logic for Each Pair

(A) Adenine → (II)

Adenine is a purine base (double ring).

It contains an amino group (–NH₂) at position 6 and no carbonyl group.

This uniquely matches structure (II).

(B) Cytosine → (IV)

Cytosine is a pyrimidine (single ring).

It has one amino group and one carbonyl group.

This corresponds to structure (IV).

(C) Thymine → (I)

Thymine is a pyrimidine base.

It contains two carbonyl groups and a methyl group (–CH₃).

This matches structure (I).

(D) Guanine → (III)

Guanine is a purine.

It contains one carbonyl group and one amino group.

This matches structure (III).