Q.2 Enzyme responsible for relaxing positive supercoiling of DNA in prokaryotes
(1) Type I topoisomerase
(2) DNA gyrase
(3) Catenase
(4) DNA ligase

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DNA supercoiling affects replication and transcription in prokaryotes. Positive supercoiling (overwinding) hinders these processes, so cells use specific enzymes to manage topology. Identifying the enzyme relaxing positive supercoiling is key for genetics and microbiology exams.

Correct Answer: Option (2) DNA gyrase

DNA gyrase, a type II topoisomerase (Topoisomerase II), uniquely introduces negative supercoils while relaxing positive ones in prokaryotes like E. coli. It uses ATP to cut both DNA strands, pass another segment through, and reseal—converting positive to negative supercoils for compact, accessible DNA.

Mechanism snapshot: Gyrase binds overwound DNA, ATP hydrolysis drives strand passage, relaxing positive supercoils ahead of replication forks.

Example: Antibiotics like ciprofloxacin target gyrase, blocking bacterial DNA replication.

Why Other Options Are Incorrect

• Option (1) Type I topoisomerase
  Relaxes both positive and negative supercoils by single-strand nicks (no ATP needed), but cannot introduce negative supercoils or efficiently handle positive supercoiling during rapid prokaryotic replication.

• Option (3) Catenase
  Not a standard enzyme name; may confuse with topoisomerase IV (decatenase), which unknots daughter DNA catenanes post-replication but doesn’t primarily relax positive supercoils.

• Option (4) DNA ligase
  Joins DNA breaks (e.g., Okazaki fragments) by forming phosphodiester bonds—no role in supercoiling relaxation; it seals nicks, not manages topology.

Quick Comparison Table

This enzyme relaxing positive supercoiling ensures bacterial survival. Vital for understanding quinolone antibiotics in medicine.