Q.42 Given below are two statements: one is labelled as Assertion-A and the other is labelled as Reason-R

Assertion-A: Eubacteria are sensitive to chloramphenicol

Reason-R: All bacteria are sensitive to chloramphenicol

In the light of the above statements, choose the correct answer from the options given below:

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

   A is true but R is false. Eubacteria (true bacteria) generally show sensitivity to chloramphenicol as it targets their 70S ribosomes, but not all bacteria are sensitive—Archaea resist it.

   Question Analysis

   Chloramphenicol inhibits peptidyl transferase on 70S ribosomes typical of eubacteria. Assertion A holds for wild-type strains without resistance genes; Reason R overgeneralizes by including archaea and resistant mutants.

   Option Breakdown

   • Both A and R true, R explains A: Incorrect. R false since archaea (e.g., methanogens) unaffected; R doesn’t explain A specificity.

   • Both A and R true, R NOT explanation: Incorrect. R false—many bacteria resist via cat genes encoding acetyltransferase.

   • A correct, R not correct: Correct. Eubacteria sensitive generally (minus resistant strains); “all bacteria” excludes archaea resistance.

   • A not correct, R correct: Incorrect. A valid for typical eubacterial protein synthesis inhibition.

   Correct Answer: A is correct but R is not correct.

   Eubacteria are sensitive to chloramphenicol, which binds their ribosomal 50S subunit to halt translation. This specificity fails for “all bacteria” due to archaeal 70S structural differences and resistance mechanisms.

   Antibiotic Mechanism

   Chloramphenicol acetyltransferase (CAT) confers resistance in many eubacterial strains, but wild-type eubacteria lack this. Archaea resist inherently via distinct peptidyl transferase centers.

   Option	Validity	Rationale
   Both true, R explains	False	R overgeneralizes archaea
   Both true, R not explain	False	R false due resistance
   A true, R false	True	Eubacteria targeted specifically
   A false, R true	False	A holds for non-resistant strains

   Exam Strategy

   NCERT notes chloramphenicol’s broad eubacterial efficacy. R fails universal claim—key distinction for CSIR-NET assertion questions.