A DNA molecule having labelled thymidine in one of its strand is allowed to replicate in a radioactive medium. Calculate the number of strands with labelled thymine in the DNA after three successive divisions.

Options

A. 15
B. 13
C. 3
D. 16

Answer

To solve the problem of calculating the number of DNA strands with labeled thymidine after three successive divisions, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- A DNA molecule consists of two strands. In this case, one of the strands is labeled with radioactive thymidine.

2. **Nature of DNA Replication**:
- DNA replication is semi-conservative, meaning that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

3. **Initial Condition**:
- Initially, we have one labeled strand (with radioactive thymidine) and one unlabeled strand. So, we start with:
- 1 labeled strand
- 1 unlabeled strand

4. **First Division**:
- After the first replication, each strand will serve as a template. The labeled strand will produce one new labeled strand, and the unlabeled strand will produce one new unlabeled strand.
- Result after the first division:
- Total strands = 4 (2 original strands each replicated)
- Labeled strands = 3 (1 original labeled + 2 new labeled from the labeled strand)
- Unlabeled strands = 1 (the original unlabeled strand remains unlabeled)

5. **Second Division**:
- In the second division, all strands will replicate again. The three labeled strands will produce three new labeled strands, while the one unlabeled strand will produce one new unlabeled strand.
- Result after the second division:
- Total strands = 8 (4 from the first division each replicated)
- Labeled strands = 7 (3 original labeled + 4 new labeled from the labeled strands)
- Unlabeled strands = 1 (the original unlabeled strand remains unlabeled)

6. **Third Division**:
- In the third division, the seven labeled strands will produce seven new labeled strands, while the one unlabeled strand will again produce one new unlabeled strand.
- Result after the third division:
- Total strands = 16 (8 from the second division each replicated)
- Labeled strands = 15 (7 original labeled + 8 new labeled from the labeled strands)
- Unlabeled strands = 1 (the original unlabeled strand remains unlabeled)

7. **Final Count**:
- After three divisions, we have a total of 16 strands, out of which 15 strands are labeled with radioactive thymidine.

### Conclusion:
- The final answer is that after three successive divisions, **15 strands of DNA will have labeled thymidine**.