A patient receiving ciprofloxacin for complicated urinary tract infection asks how the drug works. The target enzymes are bacterial type II topoisomerases. Their physiological function that the drug blocks is:

• A Unwinding of the parental duplex ahead of the replication fork
• B Joining adjacent Okazaki fragments after primer removal
• C Synthesising RNA primers required to start Okazaki fragments
• D Relaxing negative supercoils by cutting both strands of DNA and passing a segment through the break ✓

Explanation

DNA gyrase, the bacterial type II topoisomerase targeted by fluoroquinolones, introduces transient double-strand breaks to pass another DNA segment through, thereby relieving positive supercoils and maintaining negative supercoiling behind and ahead of the fork. Unwinding at the fork is done by helicase, primers are made by primase, and nick sealing is performed by DNA ligase, none of which are quinolone targets. Quinolones trap the cleaved covalent enzyme-DNA intermediate.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.