50. Match the antibiotics in Group I with their modes of action in Group II.

(A) P-1, Q-2, R-3, S-5
(B) P-3, Q-4, R-1, S-2
(C) P-3, Q-2, R-1, S-4
(D) P-1, Q-4, R-3, S-2

Step-by-Step Matching With Explanation

P) Chloramphenicol → Inhibits protein synthesis by acting on 50S ribosomal subunit (3)

• Chloramphenicol binds to the 50S ribosomal subunit.

• It inhibits peptidyl transferase, preventing peptide bond formation.

• Effective against a broad spectrum of bacteria.

✅ Correct match: P–3

Q) Rifampicin → Interferes with RNA polymerase activity (4)

• Rifampicin binds to DNA-dependent RNA polymerase.

• It blocks transcription initiation.

• Widely used in tuberculosis treatment.

✅ Correct match: Q–4

R) Tetracycline → Inhibits protein synthesis by acting on 30S ribosomal subunit (1)

• Tetracycline binds to the 30S ribosomal subunit.

• It prevents attachment of aminoacyl-tRNA to the A-site.

• Bacteriostatic in nature.

✅ Correct match: R–1

S) Quinolone → Interferes with DNA replication by inhibiting DNA gyrase (2)

• Quinolones inhibit DNA gyrase (topoisomerase II) and topoisomerase IV.

• This blocks DNA replication and transcription.

• They are bactericidal antibiotics.

✅ Correct match: S–2

✅ Final Correct Answer

Option (B)

P–3, Q–4, R–1, S–2

Introduction

The ability to match the antibiotics with their modes of action is a fundamental concept in microbiology and pharmacology. This topic is frequently tested in competitive exams such as GATE, CSIR-NET, CUET-PG, NEET-PG, and university assessments. Understanding how antibiotics target ribosomes, enzymes, and nucleic acid synthesis helps in both exam success and clinical reasoning.

Explanation of Antibiotic–Mechanism Matching

Chloramphenicol

Chloramphenicol inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, specifically blocking peptidyl transferase activity.

➡ Mode of action: Inhibits protein synthesis at 50S ribosome

Rifampicin

Rifampicin interferes with transcription by binding to RNA polymerase, preventing RNA chain initiation. It is a cornerstone drug in tuberculosis therapy.

➡ Mode of action: Inhibits RNA polymerase activity

Tetracycline

Tetracycline prevents protein synthesis by binding to the 30S ribosomal subunit, blocking aminoacyl-tRNA attachment.

➡ Mode of action: Inhibits protein synthesis at 30S ribosome

Quinolones

Quinolones inhibit DNA gyrase and topoisomerase IV, enzymes essential for DNA replication and supercoiling.

➡ Mode of action: Inhibits DNA replication via DNA gyrase inhibition

Correct Matching Summary

Final Answer

✅ Option (B): P–3, Q–4, R–1, S–2

Exam Tip

30S → Tetracycline, 50S → Chloramphenicol, RNA polymerase → Rifampicin, DNA gyrase → Quinolones
Memorizing these core associations helps solve multiple MCQs quickly and accurately.