Mutagens which are effective on replicating DNA only are :-

Options

A. Base analogues
B. Alkylating agents
C. `HNO_(2)`
D. All the above

Answer

To determine which mutagens are effective on replicating DNA only, let's analyze the options provided: base analogues, alkylating agents, and HNO2 (nitrous acid).

### Step-by-Step Solution:

1. **Understanding Mutagens**:
- Mutagens are substances that cause mutations in DNA. They can alter the genetic information stored in DNA, leading to changes in the organism.

2. **Base Analogues**:
- Base analogues are compounds that resemble the normal bases of DNA (purines and pyrimidines).
- When incorporated into DNA during replication, they can mispair with the complementary bases.
- For example, 5-bromouracil is a base analogue that mimics thymine. It can pair with adenine during replication but can change to a structure resembling cytosine, leading to a mutation when it pairs with guanine instead of thymine.

3. **Alkylating Agents**:
- Alkylating agents are chemicals that add alkyl groups to DNA bases, particularly guanine.
- This modification can convert guanine to 7-alkyl guanine, which can mispair with thymine instead of cytosine during DNA replication, resulting in mutations.

4. **HNO2 (Nitrous Acid)**:
- Nitrous acid acts as a deaminating agent, removing amino groups from bases.
- For instance, it can convert adenine to hypoxanthine, which then pairs with cytosine instead of thymine. Similarly, cytosine can be converted to uracil, which pairs with adenine instead of guanine.
- This alteration also leads to mutations during DNA replication.

5. **Conclusion**:
- All three types of mutagens (base analogues, alkylating agents, and HNO2) are effective on replicating DNA because they all cause changes in base pairing during DNA replication, leading to mutations.
- Therefore, the correct answer is **all of the above**.

### Final Answer:
**All of the above** (base analogues, alkylating agents, HNO2).

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