A DNA molecule having labelled thymidine in one of its strand is allowed ti 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 how many strands of DNA will have labeled thymidine after three successive divisions, we will follow these steps:

### Step 1: Understand the Initial Condition
We start with a DNA molecule that has one strand labeled with radioactive thymidine. This means that in the first round of replication, one of the parental strands will have the labeled thymidine.

**Hint:** Identify the starting condition of the DNA molecule before replication begins.

### Step 2: First Round of Replication
During the first round of replication, the DNA will separate into two strands. One strand will be the original (parental) strand with labeled thymidine, and the other will be a newly synthesized strand that will not have any label. Thus, after the first division, we have:
- 1 strand with labeled thymidine (parental)
- 1 strand without labeled thymidine (newly synthesized)

**Hint:** Remember that DNA replication is semi-conservative, meaning each new DNA molecule consists of one old strand and one new strand.

### Step 3: Second Round of Replication
In the second round of replication, each of the two strands from the first division will serve as a template. The strand with labeled thymidine will produce one labeled daughter strand and one new strand. The newly synthesized strand from the first division will also produce one labeled daughter strand and one new strand. Thus, after the second division, we have:
- 2 strands with labeled thymidine (from the original labeled strand)
- 2 strands without labeled thymidine (newly synthesized strands)

**Hint:** Count how many strands are produced after each division and how the labeling is distributed.

### Step 4: Third Round of Replication
In the third round of replication, each of the four strands from the second division will serve as a template. The two strands that were labeled in the second division will produce two more labeled strands, and the two newly synthesized strands will produce two new strands. Thus, after the third division, we have:
- 4 strands with labeled thymidine (2 from the first labeled strand and 2 from the second division)
- 4 strands without labeled thymidine (newly synthesized strands)

**Hint:** Keep track of how many strands are labeled after each division.

### Step 5: Total Count of Strands
After three divisions, we have a total of 8 strands (2^3 = 8). Out of these, 7 strands will have labeled thymidine, as the original labeled strand continues to contribute to the labeled strands in each division.

**Hint:** Use the formula for the total number of strands after n divisions, which is 2^n, where n is the number of divisions.

### Conclusion
After three successive divisions, the total number of strands with labeled thymidine is **15**.

**Final Answer:** 15 strands with labeled thymidine.