DNA topology: which enzyme introduces negative supercoils into bacterial double-stranded DNA? Select the enzyme whose action leads to supercoiling of dsDNA in bacteria.

Introduction / Context:
DNA topology is controlled by topoisomerases that relieve or introduce supercoils. In bacteria, negative supercoiling aids in genome compaction and replication. This question probes recognition of the specific enzyme responsible.

Given Data / Assumptions:

• Bacterial chromosomes are circular and highly compacted.
• Topoisomerases alter linking number by introducing transient breaks.
• Nomenclature in the options contrasts real versus fictitious enzyme names.

Concept / Approach:
DNA gyrase (topoisomerase II) introduces negative supercoils using ATP and can relax positive supercoils ahead of replication forks. It is the target of quinolone and fluoroquinolone antibiotics (for example, ciprofloxacin), which inhibit DNA replication by trapping gyrase–DNA complexes.

Step-by-Step Solution:
Recall that “gyrase” is the canonical bacterial enzyme adding negative supercoils.Recognize “pyrase” and “RNA gyrase/pyrase” are not standard enzymes in DNA topology.Select DNA gyrase as the correct choice.

Verification / Alternative check:
Genetics and microbiology texts describe gyrase subunits GyrA and GyrB and their inhibition by quinolones; this confirms its role in supercoiling.

Why Other Options Are Wrong:
They are either fictitious or not relevant to introducing negative supercoils (helicase unwinds DNA but does not change linking number directly).

Common Pitfalls:
Confusing gyrase (Type II) with Topo I. Topo I relieves negative supercoils; gyrase introduces them.

Final Answer:
DNA gyrase (a type II topoisomerase).