Sequence of enzyme acting in DNA replication is

Options

A. Helicase, topoisomerase, DNA polymerase, ligase
B. Topoisomerase, helicase, DNA polymerase, ligase
C. DNA polymerase, helicase, ligase, topoisomerase
D. Ligase, helicase, topoisomerase, DNA polymerase

Answer

**Step-by-Step Solution:**

1. **Understanding the Replication Fork:**
- DNA replication occurs at a structure known as the replication fork, where the double-stranded DNA unwinds to allow for the synthesis of new strands.

2. **Role of Helicase:**
- The first enzyme to act during DNA replication is **helicase**. It unwinds the DNA double helix and separates the two strands by breaking the hydrogen bonds between the base pairs. This process requires energy, which is provided by ATP.

3. **Function of Topoisomerase:**
- After helicase unwinds the DNA, the next enzyme to act is **topoisomerase**. This enzyme alleviates the torsional strain that builds up ahead of the replication fork due to the unwinding of the DNA. It prevents the DNA strands from becoming tangled or supercoiled.

4. **Action of DNA Polymerase:**
- Following the action of topoisomerase, **DNA polymerase** comes into play. This enzyme synthesizes new DNA strands by adding nucleotides complementary to the template strand. It requires a short RNA primer to initiate the synthesis.

5. **Role of Ligase:**
- Finally, the enzyme **ligase** is responsible for joining the Okazaki fragments on the lagging strand. It seals the nicks in the sugar-phosphate backbone, ensuring that the newly synthesized DNA strands are continuous.

6. **Sequence of Enzymes:**
- Therefore, the correct sequence of enzymes acting during DNA replication is:
1. Helicase
2. Topoisomerase
3. DNA Polymerase
4. Ligase

**Final Answer:** The sequence of enzymes acting in DNA replication is: **Helicase → Topoisomerase → DNA Polymerase → Ligase.**

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