17. The following statements were made about bacterialDNA gyrase, which introduces negative supercoiling inDNA.
    A. If gyrase activity is inhibited, there is an increase inreplication initiation at the origin.
    B. If gyrase activity is inhibited, there is a decrease inreplication initiation at the origin.
    C. Gyrases are usually ATP-dependent.
    D. When gyrase introduces negative supercoils, itdecreases, the linking number of a DNA loop by two.
    Which of the above statement(s) is/are NOT true?

(1) C and D(2) only B
(3) B and C(4) only A

Introduction

Bacterial DNA gyrase is a unique enzyme that introduces negative supercoils into DNA, facilitating essential processes such as replication and transcription. Understanding its mechanism, energy requirements, and influence on DNA replication initiation is crucial in molecular biology and antibiotic development. Several statements about gyrase activity are often discussed, but not all are accurate. This article clarifies which statements about DNA gyrase are not true based on current scientific knowledge.

The Statements Under Review

A. If gyrase activity is inhibited, there is an increase in replication initiation at the origin.
B. If gyrase activity is inhibited, there is a decrease in replication initiation at the origin.
C. Gyrases are usually ATP-dependent.
D. When gyrase introduces negative supercoils, it decreases the linking number of a DNA loop by two.

Evaluating Each Statement

Statement A: If gyrase activity is inhibited, there is an increase in replication initiation at the origin.

• This is not true. Inhibition of gyrase reduces negative supercoiling, which is necessary to relieve topological stress and facilitate replication initiation. Therefore, inhibition typically decreases replication initiation.

Statement B: If gyrase activity is inhibited, there is a decrease in replication initiation at the origin.

• This is true. Gyrase activity is essential for maintaining DNA topology conducive to replication. Its inhibition hampers initiation.

Statement C: Gyrases are usually ATP-dependent.

• This is true. DNA gyrase is an ATP-dependent enzyme; it hydrolyzes ATP to introduce negative supercoils.

Statement D: When gyrase introduces negative supercoils, it decreases the linking number of a DNA loop by two.

• This is true. Each catalytic cycle of gyrase changes the DNA linking number by -2 through a mechanism involving double-strand breaks and strand passage.

Which Statements Are NOT True?

• Statement A is false.

• Statements B, C, and D are true.

Correct Answer

(4) only A

Summary Table

Biological Significance

DNA gyrase plays a vital role in bacterial DNA metabolism by introducing negative supercoils, which facilitates replication fork progression and transcription. Its ATP-dependent mechanism and precise control over DNA topology make it an attractive target for antibiotics. Understanding which statements about gyrase are accurate helps in studying bacterial replication and designing gyrase inhibitors.

Keywords

DNA gyrase, bacterial gyrase, ATP-dependent enzyme, negative supercoiling, DNA replication initiation, linking number, gyrase inhibition, replication origin, molecular biology, topoisomerase II

Conclusion

Among the given statements about bacterial DNA gyrase, only statement A is incorrect. Inhibiting gyrase does not increase replication initiation; rather, it decreases it. Gyrase’s ATP dependence and its ability to reduce the DNA linking number by two per cycle are well-established facts critical for DNA replication and cellular function.