Question

Topoisomerases A. regulate the level of superhelicity of DNA in cells. B. always break only one strand of DNA. C. can create but not remove supercoils. D. must hydrolyze ATP for their action. E. of the subclass gyrases, introduce negative superhelices in eukaryotic DNA.

Short answer

Answer: A. Topoisomerases regulate the level of superhelicity of DNA in cells.

Step-by-step solution

Statement A

Topoisomerases regulate the level of superhelicity of DNA in cells. This statement is correct. Topoisomerases are enzymes that play a vital role in changing the degree of supercoiling of DNA during various cellular processes, including replication, transcription, and recombination. Supercoiled DNA is the result of the DNA strand being twisted or coiled upon itself in a compact structure to fit into cells. Topoisomerases manage this level of supercoiling by breaking and resealing DNA strands, thereby allowing the DNA to unwind and be accessible for biological functions.

Statement B

Topoisomerases always break only one strand of DNA. This statement is false. There are two types of topoisomerases: Type I and Type II. Type I topoisomerases break only one strand of DNA, while Type II topoisomerases simultaneously break both strands of DNA. Therefore, it is incorrect to assert that all topoisomerases only break a single strand of DNA.

Statement C

Topoisomerases can create but not remove supercoils. This statement is false. Topoisomerases can both create and remove supercoils. By cleaving and resealing DNA strands, topoisomerases can increase or decrease the level of supercoiling, depending on the cell's requirements.

Statement D

Topoisomerases must hydrolyze ATP for their action. This statement is partially true. Type II topoisomerases require ATP hydrolysis to perform their functions, but Type I topoisomerases do not require ATP for their activity. Consequently, it is incorrect to assert that all topoisomerases need ATP hydrolysis for their action.

Statement E

Topoisomerases of the subclass gyrases introduce negative superhelices in eukaryotic DNA. This statement is false. While it is true that DNA gyrase, a Type II topoisomerase, introduces negative supercoils into DNA, this enzyme is found primarily in bacteria and not eukaryotic cells. Eukaryotic cells have other Type II topoisomerases that fulfill a similar function, but gyrase itself is not considered a eukaryotic enzyme. In conclusion,

Correct Statement

The correct answer to the exercise is A. Topoisomerases regulate the level of superhelicity of DNA in cells. This statement is accurate, as topoisomerases control the degree of supercoiling of DNA to ensure proper cellular functions, including DNA replication, transcription, and recombination.