Options:

(A) P-2, Q-1, R-3, S-4

(B) P-2, Q-1, R-4, S-3

(C) P-4, Q-3, R-1, S-2

(D) P-3, Q-4, R-2, S-1

Introduction

Antibiotics inhibit microbial growth by targeting essential cellular processes, especially protein synthesis at the ribosome.
This question evaluates your knowledge of ribosomal subunits (30S, 50S, 80S) and how different antibiotics interfere with translation in prokaryotes and eukaryotes.

Correct Answer

✅ Option (B): P-2, Q-1, R-4, S-3

Detailed Explanation of Each Antibiotic

1. Tetracyclines (P → 2)

• Bind to the 30S ribosomal subunit

• Block the A-site, preventing aminoacyl-tRNA attachment

• Result: Inhibition of elongation during protein synthesis

✔ Correct match: P-2

2. Chloramphenicol (Q → 1)

• Acts on the 50S ribosomal subunit

• Inhibits peptidyl transferase activity

• Prevents peptide bond formation

✔ Correct match: Q-1

3. Cycloheximide (R → 4)

• Inhibits eukaryotic protein synthesis

• Targets 80S ribosome

• Blocks peptidyl transferase activity

✔ Correct match: R-4

4. Streptomycin (S → 3)

• Binds to 30S ribosomal subunit

• Causes misreading of genetic code

• Also inhibits initiation of protein synthesis

✔ Correct match: S-3

Why Option (B) Is Correct

All mappings align perfectly with Option (B).

Why Other Options Are Incorrect

❌ Option (A)

• Cycloheximide does not misread genetic code

• Streptomycin does not block peptidyl transferase

❌ Option (C)

• Tetracyclines do not act on 80S ribosome

• Chloramphenicol does not misread genetic code

❌ Option (D)

• Streptomycin does not block peptidyl transfer

• Tetracyclines do not inhibit initiation

Exam Memory Tricks

🧠

• TETRA = A-site

• CHLORO = Peptide bond

• STREP = Strange proteins (misreading)

• CYCLO = Cycles stop in eukaryotes (80S)

Conclusion

Protein synthesis inhibitors are a high-yield topic in microbiology and molecular biology exams.

✅ Correct answer: Option (B) — P-2, Q-1, R-4, S-3