If 50 heavy DNA `(N^(15))` molecules are replicated 2 times in `N^(14)` medium

Options

A. 100 half heavy and half light and 100 light DNA molecules
B. 100 heavy and 100 light DNA molecules
C. 50 heavy and 150 light DNA molecules
D. none of the above

Answer

To solve the problem of how many DNA molecules of each type are produced after two rounds of replication of 50 heavy DNA molecules in N14 medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- We start with 50 heavy DNA molecules labeled as N15.

2. **First Round of Replication**:
- In the first round of replication, each of the 50 heavy DNA (N15) molecules replicates in the presence of N14 medium. According to the semi-conservative model of DNA replication, each new DNA molecule consists of one old (parent) strand and one new (daughter) strand.
- Therefore, after the first replication, we will have:
- 50 molecules with one heavy strand (N15) and one light strand (N14). These are called hybrid DNA molecules.
- Total after the first replication: 50 hybrid DNA molecules (N15/N14).

3. **Calculating Total After First Replication**:
- Total DNA molecules after the first replication = 50 (hybrid N15/N14).

4. **Second Round of Replication**:
- Now, we have 50 hybrid DNA molecules (N15/N14) that will again replicate in N14 medium.
- Each hybrid DNA molecule (N15/N14) will produce:
- One strand from the original hybrid (which is N15) and one new strand from the N14 medium.
- This means:
- Each hybrid DNA (N15/N14) will produce two new DNA molecules:
- One hybrid (N15/N14) and one light (N14/N14).
- Therefore, from 50 hybrid molecules, we will get:
- 50 hybrid molecules (N15/N14) + 50 light molecules (N14/N14).

5. **Calculating Total After Second Replication**:
- Total DNA molecules after the second replication:
- 50 hybrid (N15/N14) + 50 light (N14/N14) = 100 total DNA molecules.

6. **Final Count of DNA Types**:
- After the second round of replication, we have:
- 50 hybrid (half heavy, half light) DNA molecules.
- 50 light DNA molecules.

### Conclusion:
- The final result is:
- 50 half-heavy (N15/N14) and 50 light (N14/N14) DNA molecules.

### Answer:
- The correct option is **A: 100 half heavy and half light and 100 light DNA molecules**.