If Meselson and Stahl's experiment is continued for four generations in bacteria, the ratio of `overset(15)N//overset(15)N: overset(15)N//overset(14):overset(14)//overset(14)N` containing DNA in the fourth generation would be

Options

A. `1:1:0`
B. `1:4:0`
C. `0:1:3`
D. `0:1:7`

Explanation

To solve the question regarding the ratio of DNA containing different nitrogen isotopes after four generations in Meselson and Stahl's experiment, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Setup**:
- In the beginning, all DNA in the bacteria is labeled with heavy nitrogen (15N).

2. **First Generation**:
- When the bacteria replicate, each strand of the original heavy DNA (15N) pairs with a new light nitrogen (14N) strand. Thus, all DNA in the first generation will be hybrid (15N/14N).
- Ratio after 1st generation: 0 (15N/15N) : 1 (15N/14N) : 0 (14N/14N).

3. **Second Generation**:
- In the second generation, the hybrid DNA (15N/14N) will again replicate. Each hybrid DNA will produce one strand of 15N and one strand of 14N, while the light DNA (14N/14N) will produce only light strands.
- The resulting DNA types will be:
- 50% hybrid (15N/14N)
- 50% light (14N/14N)
- Ratio after 2nd generation: 0 (15N/15N) : 1 (15N/14N) : 1 (14N/14N).

4. **Third Generation**:
- In the third generation, the DNA will again replicate. The hybrid (15N/14N) will produce 25% hybrid (15N/14N) and 75% light (14N/14N).
- The resulting DNA types will be:
- 25% hybrid (15N/14N)
- 75% light (14N/14N)
- Ratio after 3rd generation: 0 (15N/15N) : 1 (15N/14N) : 3 (14N/14N).

5. **Fourth Generation**:
- In the fourth generation, the DNA will replicate again. The hybrid DNA (15N/14N) will produce 12.5% hybrid (15N/14N) and 87.5% light (14N/14N).
- The resulting DNA types will be:
- 12.5% hybrid (15N/14N)
- 87.5% light (14N/14N)
- Ratio after 4th generation: 1 (15N/15N) : 1 (15N/14N) : 7 (14N/14N).

### Final Ratio:
After four generations, the ratio of DNA types will be:
- 1 (15N/15N) : 7 (15N/14N) : 7 (14N/14N).

### Conclusion:
The final answer is a ratio of 1 : 7 : 7.