If a long linear dsDNA having blunt ends having.
5' GAATTC3' 3'CTTAG5' sequences at four places is cleaved by using EcoRI, how many DNA fragments are formed with cohesive ends on both sides

Options

A. 5
B. 4
C. 3
D. 2

Answer

To solve the problem, we need to determine how many DNA fragments are formed with cohesive ends on both sides after cleaving a long linear double-stranded DNA (dsDNA) at specific restriction sites using the EcoRI enzyme.

### Step-by-Step Solution:

1. **Identify the Restriction Site**:
The sequence recognized by EcoRI is "GAATTC". According to the question, this sequence appears at four different locations in the long linear dsDNA.

2. **Understand the Cleavage Mechanism**:
EcoRI cuts the DNA between the G and A in the sequence "GAATTC". Thus, when EcoRI acts on this sequence, it will create sticky (cohesive) ends on both sides of the cut.

3. **Determine the Number of Cuts**:
Since the sequence "GAATTC" appears four times in the DNA, EcoRI will make cuts at each of these four locations.

4. **Calculate the Number of Fragments**:
Each cut will divide the DNA into fragments. The formula to calculate the number of fragments formed from n cuts is:
\[ \text{Number of fragments} = n + 1 \]
Here, n = 4 (the number of cuts), so:
\[ \text{Number of fragments} = 4 + 1 = 5 \]

5. **Identify Fragments with Cohesive Ends**:
- The first fragment (A) will have a cohesive end on one side and a blunt end on the other side (since it is the first fragment).
- The second (B), third (C), and fourth (D) fragments will have cohesive ends on both sides.
- The last fragment (E) will have a blunt end on one side and a cohesive end on the other side.

6. **Count the Fragments with Cohesive Ends on Both Sides**:
From the analysis:
- Fragments with cohesive ends on both sides: B, C, D (3 fragments).
- Fragments with blunt ends: A and E (2 fragments).

### Final Answer:
Thus, the number of DNA fragments formed with cohesive ends on both sides is **3**.

---