E.coli with dsDNA labeled with radioactive thymidine, allowed to replicates in a simple nutrient medium for two - generation. What will be the percentage of bacteria containing only non-radioactive DNA ?

Options

A. `12.5%`
B. 0.0625
C. 0.25
D. 0.5

Answer

To solve the problem, we need to analyze the replication process of E. coli with double-stranded DNA labeled with radioactive thymidine over two generations.

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- We start with E. coli that has double-stranded DNA, where both strands are labeled with radioactive thymidine. This means that both strands of the DNA are radioactive.

2. **First Generation Replication**:
- When E. coli replicates, it follows the semi-conservative model of DNA replication. This means that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.
- In the first generation, the two parental strands (radioactive) will separate and each will serve as a template for the synthesis of a new complementary strand using the non-radioactive thymidine from the nutrient medium.
- After the first generation, we will have:
- 2 hybrid DNA molecules (1 radioactive strand + 1 non-radioactive strand).

3. **Second Generation Replication**:
- In the second generation, the two hybrid DNA molecules will again separate. Each of these strands will serve as a template for new strand synthesis.
- The two radioactive strands will again separate and each will be paired with a new non-radioactive strand.
- The two non-radioactive strands from the first generation will also separate and will pair with new non-radioactive strands.
- After the second generation, we will have:
- 2 hybrid DNA molecules (1 radioactive strand + 1 non-radioactive strand).
- 2 non-radioactive DNA molecules (both strands are non-radioactive).

4. **Calculating the Percentage of Non-Radioactive DNA**:
- In total, there are 4 DNA molecules after two generations:
- 2 hybrid (radioactive + non-radioactive)
- 2 non-radioactive (non-radioactive + non-radioactive)
- Therefore, the percentage of bacteria containing only non-radioactive DNA is:
\[ \text{Percentage of non-radioactive DNA} = \left( \frac{\text{Number of non-radioactive DNA}}{\text{Total number of DNA}} \right) \times 100 = \left( \frac{2}{4} \right) \times 100 = 50\% \]

### Final Answer:
The percentage of bacteria containing only non-radioactive DNA after two generations is **50%**.