What is the role of topoisomerases in eukaryotic DNA replication?

Question

BlackTom AI · Accepted Answer

Topoisomerases relieve DNA supercoiling by cutting one or both strands of the double helix, allowing the DNA to rotate or another duplex to pass through, and then resealing the break.

1. Relevant knowledge points and concepts
- DNA replication requires unwinding of the double helix by helicase at the replication fork.  
- Unwinding generates positive supercoils (overwinding) ahead of the fork that impede fork progression.  
- Topoisomerases are enzymes that remove these supercoils: Type I makes a transient single-strand nick and reseals it (no ATP required), while Type II makes transient double-strand breaks, passes another duplex through, and reseals (ATP-dependent).  
- Other replication proteins: helicase opens the helix; DNA ligase joins Okazaki fragments; origin-binding proteins (e.g., ORC in eukaryotes) recognize origins.

2. Step-by-step reasoning process
- Helicase separates parental strands to expose templates.  
- As helicase progresses, torsional strain builds ahead of the fork as positive supercoils.  
- A topoisomerase binds ahead of the fork, cleaves DNA (one or both strands), allows controlled relaxation (rotation or strand passage), then religates the phosphodiester backbone.  
- This action prevents DNA tangling and permits replication forks to advance smoothly.

3. Intermediate steps (mechanistic detail)
- Type I: enzyme forms a covalent tyrosyl-DNA linkage, allows strand swivel, reseals nick.  
- Type II: enzyme covalently links to both strands, passes another duplex through the break, hydrolyzes ATP, and religates both strands.

4. Why the correct answer is correct
The correct option states that topoisomerases “cut, uncoil and reseal the double stranded DNA,” which exactly describes their biochemical mechanism of transient cleavage, relaxation of supercoils (uncoiling), and ligation.

5. Why other options are incorrect
- “Open up the double stranded DNA at the replication fork” describes helicase, not topoisomerase.  
- “Join the Okazaki fragments together with phosphodietster bonds” is the function of DNA ligase.  
- “Bind to the origin of replication sites” describes origin recognition complexes (ORC) and initiator proteins, not topoisomerases.

6. Conclusion
Therefore, the correct answer is: Topoisomerise enzymes cut, uncoil and reseal the double stranded DNA. This matches the provided answer.

What is the role of topoisomerases in eukaryotic DNA replication?单项选择题

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