156. Which one of the following enzymes help the DNA to overcome the tension generated by the
unwinding during replication?
A. Nickase
B. Helicase
C. Topoisomerase
D. DNA ligase

The Role of Topoisomerase in DNA Replication

DNA replication is a critical process in the cell cycle, where the genetic material is copied to ensure that both daughter cells receive an identical set of DNA. As the DNA double helix unwinds during replication, tension builds up ahead of the replication fork, potentially causing the DNA to become over-wound or tangled. To address this tension and ensure smooth replication, cells rely on certain enzymes.

One key enzyme involved in relieving this tension is topoisomerase.

Which Enzyme Helps DNA Overcome the Tension Generated by Unwinding During Replication?

The correct answer is C. Topoisomerase.

Topoisomerase is responsible for alleviating the torsional strain that builds up in the DNA ahead of the replication fork. As the helicase unwinds the DNA, the two strands of the helix are separated, and this generates supercoiling or tension ahead of the fork. Topoisomerase works by introducing temporary single- or double-strand breaks in the DNA, allowing it to be untwisted, and then resealing the breaks after the tension is relieved.

How Does Topoisomerase Work?

Topoisomerase can be divided into two types based on the type of break it induces:

• Type I Topoisomerase: This enzyme makes a single-strand break in the DNA, which allows the DNA to rotate and relieve the tension. Afterward, the break is resealed.

• Type II Topoisomerase: This enzyme makes a double-strand break and passes another part of the DNA through the break to alleviate the tension. It also reseals the break afterward.

These actions prevent the DNA from becoming too tangled and ensure that the replication machinery can proceed smoothly.

Other Enzymes Involved in DNA Replication

• A. Nickase: A nickase is an enzyme that introduces a single-strand break in the DNA. While related to topoisomerases, it is not directly involved in relieving the tension generated during replication.

• B. Helicase: Helicase is responsible for unwinding the DNA double helix ahead of the replication fork, but it does not relieve the tension created by this unwinding.

• D. DNA Ligase: DNA ligase is responsible for sealing the nicks in the DNA after replication and repair processes, but it does not play a role in alleviating the tension caused by unwinding.

Conclusion

The correct enzyme that helps DNA overcome the tension generated by unwinding during replication is topoisomerase. By introducing temporary breaks in the DNA, topoisomerase ensures that the DNA can be untwisted without becoming over-twisted, allowing replication to proceed smoothly.