A normal DNA molecule is continuously replicate in `N^(15)` medium than what is the `%` of lighter DNA in `4^(th)` generation.

Options

A. `12.5%`
B. `25%`
C. `0%`
D. `6.25%`

Answer

To determine the percentage of lighter DNA in the 4th generation of DNA that has been continuously replicated in N15 medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Initial Condition**:
- We start with normal DNA, which is composed of N14 (the lighter isotope of nitrogen).

2. **First Generation**:
- When this DNA is placed in N15 medium, the first replication will yield two strands of DNA, both of which will be hybrid (one strand N14 and one strand N15).
- Thus, in the first generation, there are no lighter (N14) DNA strands; all DNA is either hybrid or heavier.

3. **Second Generation**:
- In the second generation, the hybrid strands from the first generation will replicate.
- Each hybrid strand will produce one N15 strand (heavy) and one N14 strand (light).
- Therefore, in the second generation, we have:
- 2 hybrid strands (N14/N15)
- 2 lighter strands (N14/N14)
- Total: 2 N14/N15 and 2 N14/N14.

4. **Third Generation**:
- In the third generation, the two types of strands from the second generation will replicate:
- The two hybrid strands (N14/N15) will produce:
- 2 N15/N15 (heavy)
- 2 N14/N15 (hybrid)
- The two lighter strands (N14/N14) will produce:
- 2 N14/N14 (lighter)
- Total: 2 N14/N14 (lighter), 2 N14/N15 (hybrid), and 2 N15/N15 (heavy).

5. **Fourth Generation**:
- In the fourth generation, the strands from the third generation will replicate:
- The two N14/N15 strands will produce:
- 2 N15/N15 (heavy)
- 2 N14/N15 (hybrid)
- The two N14/N14 strands will produce:
- 2 N14/N14 (lighter)
- However, since the medium is N15, any new strands formed will incorporate N15.
- Thus, in the fourth generation, we will have:
- 2 N15/N15 (heavy)
- 2 N14/N15 (hybrid)
- There will be no new lighter (N14/N14) strands formed because all new strands are made from N15.

6. **Conclusion**:
- In the fourth generation, there are no lighter DNA strands present.
- Therefore, the percentage of lighter DNA in the fourth generation is **0%**.

### Final Answer:
The percentage of lighter DNA in the 4th generation is **0%**.