A completely radio active DNA was allowed to replicate in non radio medium for two generations. Percentage of radioactive DNA is after process :-

Options

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

Answer

To solve the question regarding the percentage of radioactive DNA after two generations of replication in a non-radioactive medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Nature of DNA Replication**:
- DNA replication is semi-conservative. This means that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

2. **Initial Condition**:
- We start with completely radioactive DNA. This means both strands of the DNA are radioactive.

3. **First Generation of Replication**:
- When the radioactive DNA replicates in a non-radioactive medium, each of the two strands will serve as a template for the synthesis of a new strand.
- After the first round of replication, we will have two DNA molecules:
- One molecule will have one radioactive strand (from the original) and one non-radioactive strand (newly synthesized).
- The other molecule will also have one radioactive strand and one non-radioactive strand.
- Therefore, after the first generation, we have 50% of the DNA that is radioactive.

4. **Second Generation of Replication**:
- In the second generation, each of the two DNA molecules from the first generation will again replicate.
- Each of these molecules will produce two new DNA molecules:
- The molecule with one radioactive strand and one non-radioactive strand will produce:
- One molecule with one radioactive strand and one non-radioactive strand.
- One molecule with two non-radioactive strands (the radioactive strand serves as a template for a new non-radioactive strand).
- Thus, from the two original molecules, we will have:
- 2 molecules with one radioactive strand and one non-radioactive strand.
- 2 molecules with two non-radioactive strands.
- Therefore, after the second generation, we will have a total of 4 DNA molecules, with 2 of them containing one radioactive strand.

5. **Calculate the Percentage of Radioactive DNA**:
- Out of 4 total DNA molecules, 2 contain radioactive DNA.
- The percentage of radioactive DNA is therefore:
\[ \text{Percentage of radioactive DNA} = \left( \frac{\text{Number of radioactive strands}}{\text{Total strands}} \right) \times 100 = \left( \frac{2}{4} \right) \times 100 = 50\% \]

### Final Answer:
The percentage of radioactive DNA after two generations of replication in a non-radioactive medium is **50%**.

---