Question

A bacterium having radioactive thymidine in its DNA is allowed to multiply in a medium having non-radioactive thymidine for two generations. What percentage of bacteria will have radioactive thymidine in its DNA ?

• A. 100
• B. 50
• C. 75
• D. 25

Hint:

Each old radioactive strand stays in one DNA molecule, so after two generations 2 of 4 bacteria carry it, which is 50 percent.

Answer 1

The Correct Option is B

Step 1: Use semi-conservative replication:
In replication each new DNA keeps one old strand and gets one new strand.
Start with one bacterium whose DNA has both strands radioactive.

Step 2: After generation 1:
The DNA makes 2 daughter DNA molecules. Each has one radioactive strand and one non-radioactive strand.
So 2 bacteria form, and both carry some radioactivity.

Step 3: After generation 2:
Each of the 2 DNA molecules replicates again, giving 4 DNA molecules in 4 bacteria.
The 2 old radioactive strands are still there, one in each of 2 DNA molecules (each hybrid). The other 2 DNA molecules are made of two new strands, so they are not radioactive.

Step 4: Calculate the percentage:
\[ \frac{2}{4} \times 100 = 50\% \]
100 and 75 are wrong because only 2 radioactive strands exist, so at most 2 of the 4 cells can carry them. 25 would need only one cell.

Final Answer:
Half of the bacteria still have radioactive thymidine. This is option (B). \[ \boxed{50\%} \]