A patient treated with ciprofloxacin develops tendon rupture. The drug inhibits bacterial DNA gyrase. Which specific function of DNA gyrase is blocked by fluoroquinolones?
- A Relief of positive supercoils ahead of the replication fork via double-strand breakage and religation ✓
- B Unwinding of the double helix at the replication fork by breaking hydrogen bonds
- C Synthesis of short RNA primers required to initiate Okazaki fragments
- D Sealing of nicks between adjacent Okazaki fragments
Correct answer: A. Relief of positive supercoils ahead of the replication fork via double-strand breakage and religation
Explanation
DNA gyrase is a bacterial type II topoisomerase that introduces a transient double-strand break to remove positive supercoils generated ahead of the advancing fork. Fluoroquinolones stabilize the cleaved DNA-gyrase complex, blocking religation and causing lethal breaks; this explains their adverse effect on tendons, which contain type II topoisomerase in mitochondria. Helicase unwinds the helix, primase makes RNA primers, and ligase seals nicks, so these are separate enzymes from the gyrase target.
Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.