The DNA replication is semi-conservative is proved by an experiment conducted by Meselson and Stahl in 1958. To perform their experiment they use heavy nitrogen (`""^(15)N`) in E. coli. The process of replication. in living cells requires a set of enzymes. The main enzyme is DNA dependent DNA polymerase. The DNA-A dependent DNA polymerase catalyse polymerization only in one direction, that is `5 to 3'`. In eukaryotes, the replication of DNA takes place at the S-phase of the cell cycle.
During DNA replication, the breaking of H-bonds is performed by:

Options

A. Topoisomerase
B. Gyrase
C. Helicases
D. None

Explanation

To solve the question regarding which enzyme is responsible for breaking hydrogen bonds during DNA replication, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA consists of two strands that are held together by hydrogen bonds between complementary nitrogenous bases (adenine-thymine and guanine-cytosine).

2. **Recognizing the Process of DNA Replication**:
- DNA replication is a semi-conservative process where each new DNA molecule consists of one old strand and one newly synthesized strand.

3. **Identifying the Role of Enzymes in DNA Replication**:
- Several enzymes are involved in DNA replication, including DNA polymerases, helicases, and others. Each enzyme has a specific function.

4. **Focusing on the Breaking of Hydrogen Bonds**:
- During DNA replication, the two strands of the DNA double helix must be separated to allow the replication machinery to access the template strands.

5. **Identifying the Correct Enzyme**:
- The enzyme responsible for unwinding the DNA and breaking the hydrogen bonds between the base pairs is called **helicase**. Helicase moves along the DNA strands, separating them by breaking the hydrogen bonds.

6. **Conclusion**:
- Therefore, the correct answer to the question is that the breaking of hydrogen bonds during DNA replication is performed by **helicase**.

### Final Answer:
**Helicase** is the enzyme that breaks hydrogen bonds during DNA replication.

---