GATE BT 2023 • Q42
MSQ • 2 Marks
Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which of the following statement(s) is(are) TRUE about fluoroquinolone drugs?
Options:
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(A) They contain quinolone ring(s)
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(B) They inhibit RNA polymerase
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(C) They bind to bacterial topoisomerase
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(D) They bind to 23S rRNA within the 50S ribosome subunit
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2023 Q.42)**
**1. Governing Concept**:
- **Topic**: Antimicrobial Pharmacology & Bacterial Topoisomerase Inhibitors
- **Fluoroquinolones** (e.g., Ciprofloxacin, Levofloxacin, Norfloxacin) are broad-spectrum synthetic bactericidal antibiotics:
- **Mechanism of Action**: Inhibit bacterial Type II topoisomerases:
1. **DNA Gyrase** (especially GyrA subunit, primarily in Gram-negative bacteria).
2. **Topoisomerase IV** (primarily in Gram-positive bacteria).
- They trap the enzyme in a covalent complex with cleaved double-stranded DNA, preventing DNA resealing and blocking replication fork progression, triggering fatal double-strand DNA breaks.
- They do **NOT** inhibit the 30S or 50S ribosomal subunits or cell wall transpeptidases.
**2. Option Analysis**:
- **Option (A) [They inhibit bacterial DNA gyrase] is CORRECT**: Primary bactericidal mechanism is stabilizing the cleaved gyrase-DNA complex.
- **Option (B) [They bind to the 30S ribosomal subunit] is INCORRECT**: Aminoglycosides and tetracyclines bind 30S ribosomes; fluoroquinolones target topoisomerases.
- **Option (C) [They inhibit bacterial topoisomerase IV] is CORRECT**: Major secondary target essential for decatenating replicated daughter chromosomes.
- **Option (D) [They inhibit cell wall peptidoglycan transpeptidase] is INCORRECT**: That is the mechanism of $\beta$-lactam antibiotics (penicillins, cephalosporins).
**3. Final Answer**:
- **Correct Options**: **A, C**
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