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)N:overset(14)N//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`

Answer

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

### Step-by-Step Solution:

1. **Understanding the Initial Setup**:
- Meselson and Stahl grew E. coli in a medium containing heavy nitrogen (15N) until it was incorporated into the DNA. After this, they switched to a medium containing light nitrogen (14N) for further generations.

2. **First Generation (G1)**:
- In the first generation, the DNA consists of one heavy strand (15N) and one light strand (14N). Thus, the resulting DNA will be:
- 1 hybrid DNA (15N/14N).

3. **Second Generation (G2)**:
- The hybrid DNA (15N/14N) will replicate in the presence of 14N. Each strand will serve as a template:
- The 15N strand will produce a 14N strand.
- The 14N strand will produce another 14N strand.
- Resulting DNA:
- 2 hybrid DNA (15N/14N).
- 2 light DNA (14N/14N).
- Total: 2 (15N/14N) + 2 (14N/14N).

4. **Third Generation (G3)**:
- The replication continues:
- Each of the 2 hybrid DNA (15N/14N) will produce:
- 1 hybrid (15N/14N) and 1 light (14N/14N).
- Each of the 2 light DNA (14N/14N) will produce:
- 2 light (14N/14N).
- Resulting DNA:
- 2 hybrid DNA (15N/14N).
- 4 light DNA (14N/14N).
- Total: 2 (15N/14N) + 4 (14N/14N).

5. **Fourth Generation (G4)**:
- The replication continues:
- Each of the 2 hybrid DNA (15N/14N) will produce:
- 1 hybrid (15N/14N) and 1 light (14N/14N).
- Each of the 4 light DNA (14N/14N) will produce:
- 4 light (14N/14N).
- Resulting DNA:
- 2 hybrid DNA (15N/14N).
- 8 light DNA (14N/14N).
- Total: 2 (15N/14N) + 8 (14N/14N).

6. **Calculating the Ratios**:
- After four generations, we have:
- 0 (15N/15N) strands (since no original heavy DNA remains).
- 2 (15N/14N) strands.
- 8 (14N/14N) strands.
- The ratio of (15N/15N) : (15N/14N) : (14N/14N) is:
- 0 : 2 : 8, which simplifies to 0 : 1 : 4.

### Final Answer:
The ratio of `N15//N15 : N15//N14 : N14//N14` in the fourth generation is **0 : 1 : 4**.