A DNA molecule in which both strands have radioactive thymidine is allowed to duplicate in an environment containing non- radioactive thymidine. What will be the exact number of DNA molecules that contains the radio active thymidine after 3 duplications -

Options

A. one
B. two
C. four
D. Eight

Answer

To solve the problem, we need to understand the semi-conservative nature of DNA replication and how radioactive thymidine is distributed during this process. Here’s a step-by-step breakdown:

### Step 1: Initial DNA Molecule
- We start with one DNA molecule where **both strands contain radioactive thymidine**. Let's denote this as **DNA1**.

### Step 2: First Replication
- During the first round of replication, the two strands of **DNA1** separate. Each strand serves as a template for the synthesis of a new strand.
- The result will be **two new DNA molecules**:
- **DNA2**: One strand from **DNA1** (radioactive) and one newly synthesized strand (non-radioactive).
- **DNA3**: The other strand from **DNA1** (radioactive) and another newly synthesized strand (non-radioactive).

After the first replication, we have:
- **DNA2**: 1 radioactive strand + 1 non-radioactive strand
- **DNA3**: 1 radioactive strand + 1 non-radioactive strand

### Step 3: Second Replication
- Now, we have **DNA2** and **DNA3**.
- **DNA2** (1 radioactive, 1 non-radioactive) will replicate, yielding:
- **DNA4**: 1 radioactive strand + 1 new non-radioactive strand
- **DNA5**: 1 non-radioactive strand + 1 new non-radioactive strand

- **DNA3** (1 radioactive, 1 non-radioactive) will replicate, yielding:
- **DNA6**: 1 radioactive strand + 1 new non-radioactive strand
- **DNA7**: 1 non-radioactive strand + 1 new non-radioactive strand

After the second replication, we have:
- **DNA4**: 1 radioactive strand + 1 non-radioactive strand
- **DNA5**: 2 non-radioactive strands
- **DNA6**: 1 radioactive strand + 1 non-radioactive strand
- **DNA7**: 2 non-radioactive strands

### Step 4: Third Replication
- Now, we will replicate the four DNA molecules:
- **DNA4** (1 radioactive, 1 non-radioactive) will yield:
- **DNA8**: 1 radioactive + 1 new non-radioactive
- **DNA9**: 1 non-radioactive + 1 new non-radioactive

- **DNA5** (2 non-radioactive) will yield:
- **DNA10**: 2 non-radioactive strands
- **DNA11**: 2 non-radioactive strands

- **DNA6** (1 radioactive, 1 non-radioactive) will yield:
- **DNA12**: 1 radioactive + 1 new non-radioactive
- **DNA13**: 1 non-radioactive + 1 new non-radioactive

- **DNA7** (2 non-radioactive) will yield:
- **DNA14**: 2 non-radioactive strands
- **DNA15**: 2 non-radioactive strands

After the third replication, we have:
- **DNA8**: 1 radioactive strand + 1 non-radioactive strand
- **DNA9**: 2 non-radioactive strands
- **DNA10**: 2 non-radioactive strands
- **DNA12**: 1 radioactive strand + 1 non-radioactive strand
- **DNA13**: 1 non-radioactive strand + 1 new non-radioactive strand
- **DNA14**: 2 non-radioactive strands
- **DNA15**: 2 non-radioactive strands

### Final Count
After 3 duplications, we have:
- **2 DNA molecules** that contain radioactive thymidine: **DNA8** and **DNA12**.

### Conclusion
The exact number of DNA molecules that contain radioactive thymidine after 3 duplications is **2**.

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