When DNA replication starts

Options

A. The leading strand produces okazaki fragments
B. The hydrogen bonds between the nucleotides of two strands break
C. The phosphodiester bonds between the adjacent nucleotides break
D. The bonds between the nitrogen base and deoxyribose sugar base break

Answer

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA consists of two strands that are held together by hydrogen bonds between the nitrogenous bases. Each strand has a directionality, indicated as 5' to 3' and 3' to 5'.

2. **Identifying the Replication Fork**:
- During DNA replication, the double helix unwinds at a specific location called the replication fork, creating two single strands of DNA.

3. **Recognizing the Leading and Lagging Strands**:
- The strand that is synthesized continuously in the direction of the replication fork is called the leading strand. The other strand, which is synthesized in short segments called Okazaki fragments, is known as the lagging strand.

4. **Evaluating the Options**:
- **Option A**: "The leading strand produces Okazaki fragments."
- This is incorrect because Okazaki fragments are produced on the lagging strand, not the leading strand.

- **Option B**: "The hydrogen bonds between the nucleotides of two strands break."
- This is correct. Before replication can occur, the hydrogen bonds between the two strands of DNA must break to separate them.

- **Option C**: "The phosphodiester bonds between the adjacent nucleotides break."
- This is incorrect. Phosphodiester bonds remain intact during replication; they connect the nucleotides within each strand.

- **Option D**: "The bonds between the nitrogen base and deoxyribose sugar break."
- This is also incorrect. These bonds do not break during replication.

5. **Conclusion**:
- The correct answer is **Option B**: "The hydrogen bonds between the nucleotides of two strands break." This is the first step that occurs when DNA replication starts.