A colony of E. coli was cultured on medium containing radioactive isotopes of nitrogen. After replication, they were transferred to a medium containing non-radioactive nitrogen. How many DNA molecules will be completely non-radioactive after the second replication? (If the bacteria replicated every 20 minutes)

Options

A. 1
B. 2
C. 3
D. 4

Answer

To solve the problem, we need to understand the process of DNA replication and how the incorporation of radioactive and non-radioactive isotopes affects the resulting DNA molecules. Here’s a step-by-step breakdown:

### Step 1: Initial Conditions
- **Starting Medium**: E. coli is cultured in a medium containing radioactive nitrogen isotopes.
- **After First Replication**: When the E. coli replicates its DNA, each new DNA molecule consists of one strand from the original (which is radioactive) and one newly synthesized strand (which is also radioactive).

### Step 2: First Replication Outcome
- After the first replication, we have:
- 2 DNA molecules:
- 1st DNA molecule: 1 radioactive strand (from the parent) + 1 radioactive strand (newly synthesized)
- 2nd DNA molecule: 1 radioactive strand (from the parent) + 1 radioactive strand (newly synthesized)

### Step 3: Transfer to Non-Radioactive Medium
- **New Medium**: The bacteria are then transferred to a medium containing non-radioactive nitrogen.
- **Second Replication**: During the second replication, the radioactive strands will pair with newly synthesized non-radioactive strands.

### Step 4: Second Replication Outcome
- After the second replication, we will have:
- From the first two DNA molecules (each with one radioactive and one radioactive strand):
- 1st DNA molecule: 1 radioactive strand (from the first replication) + 1 non-radioactive strand (newly synthesized)
- 2nd DNA molecule: 1 radioactive strand (from the first replication) + 1 non-radioactive strand (newly synthesized)
- This results in:
- 2 DNA molecules with one radioactive and one non-radioactive strand each.

- Additionally, the newly synthesized strands from the second replication will also create:
- 3rd DNA molecule: 1 non-radioactive strand (newly synthesized) + 1 non-radioactive strand (newly synthesized)
- 4th DNA molecule: 1 non-radioactive strand (newly synthesized) + 1 non-radioactive strand (newly synthesized)

### Step 5: Final Count of Non-Radioactive DNA Molecules
- Thus, after the second replication, we have:
- 2 DNA molecules that are radioactive (each with one radioactive and one non-radioactive strand).
- 2 DNA molecules that are completely non-radioactive (each with two non-radioactive strands).

### Conclusion
- Therefore, the number of DNA molecules that will be completely non-radioactive after the second replication is **2**.

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