E.coli with dsDNA labeled with radioactive thymidine, allowed to replicates in a simple nutrient medium for two - generation. What will be the percentage of bacteria containing only non-radioactive DNA ?

Options

A. `12.5%`
B. 0.0625
C. 0.25
D. 0.5

Answer

To solve the problem of determining the percentage of E. coli bacteria containing only non-radioactive DNA after two generations of replication in a medium with non-radioactive thymidine, we will follow these steps:

### Step 1: Understand the Replication Process
E. coli replicates its DNA through a semi-conservative mechanism. This means that during DNA replication, each of the two strands of the original double-stranded DNA serves as a template for the synthesis of a new complementary strand. As a result, each daughter DNA molecule consists of one old (parental) strand and one newly synthesized strand.

**Hint:** Remember that in semi-conservative replication, each new DNA molecule contains one strand from the parent and one newly synthesized strand.

### Step 2: Initial Condition
Initially, all the DNA in E. coli is labeled with radioactive thymidine. This means that both strands of the double-stranded DNA are radioactive.

**Hint:** Identify that the starting DNA is fully radioactive.

### Step 3: First Generation Replication
When E. coli replicates its DNA for the first time, each of the two strands of the original DNA will separate. Each strand will serve as a template for the synthesis of a new strand. After the first generation, we will have:
- 2 DNA molecules:
- 1 with one parental (radioactive) strand and one newly synthesized (non-radioactive) strand.
- 1 with one parental (radioactive) strand and one newly synthesized (non-radioactive) strand.

Thus, after the first generation, we have:
- 2 DNA molecules with 2 radioactive strands and 2 non-radioactive strands.

**Hint:** After the first generation, each DNA molecule has one radioactive and one non-radioactive strand.

### Step 4: Second Generation Replication
Now, when these two DNA molecules replicate again in the second generation, the following occurs:
- Each of the two DNA molecules will again separate into strands.
- The radioactive strands will serve as templates for the synthesis of new strands, which will be non-radioactive.

After the second generation, we will have:
- 4 DNA molecules:
- 2 with one radioactive strand and one non-radioactive strand (from the first generation).
- 2 with both strands being non-radioactive (newly synthesized strands).

**Hint:** Each original radioactive strand produces a non-radioactive strand in the second generation.

### Step 5: Calculate the Percentage of Non-Radioactive DNA
Now, we can summarize the DNA composition after two generations:
- Total DNA strands = 4
- Non-radioactive strands = 2 (from the first generation) + 2 (newly synthesized from the second generation) = 2 non-radioactive strands out of 4 total strands.

To find the percentage of bacteria containing only non-radioactive DNA:
- Percentage = (Number of non-radioactive DNA strands / Total DNA strands) × 100
- Percentage = (2 / 4) × 100 = 50%

Thus, the percentage of bacteria containing only non-radioactive DNA after two generations is **50%** or **0.5**.

**Final Answer:** D) 0.5