If a completely radioactive double - stranded DNA molecule undergoes two rounds of replication in a non - radioactive medium, what will be the radioactive status of the four resulting molecules ?

Options

A. All four still contain radioactivity
B. Three out four contain radioactivity
C. Radioactivity is lost from all four
D. Half the number contain no radioactivity.

Answer

To solve the question regarding the radioactive status of the resulting DNA molecules after two rounds of replication in a non-radioactive medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- We start with a completely radioactive double-stranded DNA molecule. This means both strands of the DNA are radioactive.

2. **Replication Process**:
- DNA replication is semi-conservative. This means that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

3. **First Round of Replication**:
- In the first round of replication, the radioactive DNA (let's denote it as R) will replicate in a non-radioactive medium (denote non-radioactive as NR).
- After the first round, each resulting DNA molecule will have one radioactive strand and one non-radioactive strand. Thus, we will have two types of molecules:
- Molecule 1: R (radioactive) + NR (non-radioactive)
- Molecule 2: R (radioactive) + NR (non-radioactive)

4. **Second Round of Replication**:
- In the second round, both types of molecules from the first round will replicate in the same non-radioactive medium.
- For each of the two types of molecules:
- Molecule 1 (R + NR) will produce:
- One molecule with R (radioactive) + NR (non-radioactive)
- One molecule with NR (non-radioactive) + NR (non-radioactive)
- Molecule 2 (R + NR) will produce:
- One molecule with R (radioactive) + NR (non-radioactive)
- One molecule with NR (non-radioactive) + NR (non-radioactive)

5. **Resulting Molecules**:
- After the second round, we will have four resulting DNA molecules:
- Molecule A: R + NR (radioactive)
- Molecule B: NR + NR (non-radioactive)
- Molecule C: R + NR (radioactive)
- Molecule D: NR + NR (non-radioactive)

6. **Radioactive Status**:
- Out of the four resulting molecules, two will be radioactive (Molecule A and Molecule C) and two will be non-radioactive (Molecule B and Molecule D).

### Conclusion:
- Therefore, the radioactive status of the four resulting molecules is that **2 out of 4 contain radioactivity**.

### Answer:
The correct option is: **B. Three out of four contains radioactivity.**