A bacterium with completely radioactive DNA was allowed to replicate in a non- radioactive medium for two generation what `%` of the bacteria should contain radioactive DNA:-

Options

A. `100%`
B. `50%`
C. `25%`
D. `12.5%`

Answer

To solve the problem, we need to understand how DNA replication works and how the radioactive and non-radioactive strands are distributed during replication.

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- We start with a bacterium that has completely radioactive DNA. This means that both strands of the DNA are radioactive.

2. **First Generation Replication**:
- When the bacterium replicates its DNA in a non-radioactive medium, the two strands of the radioactive DNA separate.
- Each of these strands serves as a template for the synthesis of a new complementary strand, which will be non-radioactive.
- After the first round of replication, we will have two DNA molecules:
- One molecule will consist of one radioactive strand and one non-radioactive strand (Radioactive/Non-radioactive).
- The other molecule will also consist of one radioactive strand and one non-radioactive strand (Radioactive/Non-radioactive).
- Thus, after the first generation, we have 2 molecules: 2 with radioactive DNA (50% of the total).

3. **Second Generation Replication**:
- In the second generation, each of the two DNA molecules from the first generation will again replicate.
- Each of the radioactive strands will again serve as a template for a new non-radioactive strand.
- This results in:
- From the first radioactive/non-radioactive DNA molecule, we will get:
- 1 radioactive strand + 1 new non-radioactive strand.
- From the second radioactive/non-radioactive DNA molecule, we will get:
- 1 radioactive strand + 1 new non-radioactive strand.
- Therefore, after the second generation, we will have 4 DNA strands:
- 2 strands will be radioactive (from the original radioactive strands).
- 2 strands will be non-radioactive (newly synthesized strands).
- This means that out of 4 total strands, 2 are radioactive, which gives us 50% of the strands containing radioactive DNA.

4. **Final Calculation**:
- After two generations, we have 4 DNA strands in total, with 2 of them being radioactive.
- Therefore, the percentage of bacteria that contain radioactive DNA is:
\[ \text{Percentage of radioactive DNA} = \left( \frac{2 \text{ radioactive strands}}{4 \text{ total strands}} \right) \times 100 = 50\% \]

### Conclusion:
The percentage of bacteria that should contain radioactive DNA after two generations is **50%**.