When a DNA molecule with one radioactive strand undergoes repeated replications in a medium without radioactivity for five times the number of radio labelled strands in the daughter DNA molecules

Options

A. 2
B. 1
C. 16
D. 32

Answer

To solve the problem of how many radioactive strands will be present in the daughter DNA molecules after five rounds of replication, we can follow these steps:

### Step 1: Understand the Semi-Conservative Nature of DNA Replication
DNA replication is semi-conservative, meaning that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. If we start with one radioactive strand and one non-radioactive strand, each replication will produce daughter molecules that contain one radioactive strand and one non-radioactive strand.

**Hint:** Remember that in semi-conservative replication, each daughter DNA has one old strand and one new strand.

### Step 2: Analyze the First Replication
In the first round of replication, the original DNA molecule (with one radioactive strand) will replicate. The result will be two daughter DNA molecules:
- One will have the radioactive strand from the parent.
- The other will have a newly synthesized non-radioactive strand.

So after the first replication, we have:
- 1 radioactive strand in 1 daughter DNA molecule.
- 1 non-radioactive strand in the other daughter DNA molecule.

**Hint:** After the first replication, you will have two molecules: one with a radioactive strand and one without.

### Step 3: Analyze Subsequent Replications
For each subsequent replication, the process remains the same. Each daughter DNA molecule will again replicate, producing two new daughter molecules from each existing molecule.

- After the second replication, the daughter molecules will be:
- 2 radioactive strands (one from each of the two original daughter molecules).
- 2 non-radioactive strands.

Continuing this pattern, we can summarize the number of radioactive strands after each replication:
- After 1st replication: 1 radioactive strand
- After 2nd replication: 2 radioactive strands
- After 3rd replication: 4 radioactive strands
- After 4th replication: 8 radioactive strands
- After 5th replication: 16 radioactive strands

**Hint:** Each round of replication doubles the number of daughter DNA molecules, and the radioactive strands also double.

### Step 4: Conclusion
After five rounds of replication, the total number of radioactive strands in the daughter DNA molecules will be 16.

**Final Answer:** The number of radioactive strands in the daughter DNA molecules after five replications is **16**.