If DNA replication is conservative, what woul be expected distribution of labelled DNA molecule after two generations.

Options

A. 1/4 of the molecules would be `N^(15)" "N^(15)`
B. 3/4 of the molecues would be `N^(14)" "N^(14)`
C. No `N^(14)" "N^(15)` molecules would be observed
D. All the above statements are correct

Answer

To determine the expected distribution of labeled DNA molecules after two generations of conservative replication, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Conservative Replication**: In conservative replication, the original DNA molecule remains intact, and an entirely new molecule is synthesized. This means that after one round of replication, we would have one molecule of fully original DNA and one molecule of fully new DNA.

2. **Labeling the DNA**: Let's assume that the original DNA is labeled with a heavy isotope, N15. After the first generation of replication, we would have:
- One complete N15 DNA molecule (the original).
- One complete N14 DNA molecule (the newly synthesized).

3. **First Generation Distribution**: After the first generation, the distribution of DNA would be:
- 1 N15 (original)
- 1 N14 (newly synthesized)

4. **Second Generation Replication**: In the second generation, the original N15 DNA will replicate again, producing:
- One N15 molecule (original) remains unchanged.
- The N14 molecule will also replicate, producing:
- One N14 (original) and one N15 (newly synthesized from the N14 strand).

5. **Second Generation Distribution**: After the second generation, the distribution of DNA would be:
- 1 N15 (from the original)
- 2 N14 (one from the first N14 and one newly synthesized from the second N14)
- 1 N15 (newly synthesized from the second generation N14)

6. **Final Count of DNA Molecules**: Therefore, after two generations, the expected distribution of labeled DNA molecules would be:
- 1 N15/N15 (from the original)
- 2 N14/N14 (from the first N14)
- 1 N15/N14 (from the second generation)

### Summary of Distribution:
- 1 molecule of N15/N15
- 2 molecules of N14/N14
- 1 molecule of N15/N14