Which of the following is incorrect regarding gel electrophoresis?

Options

A. Agarose gel is used for separation of DNA fragments while polyacrylamide gels ate used for protein separation
B. Separated DNA fragments can be visualised with or without staining with ethidium bromide
C. The desired band of DNA containing gene of interest are extracted from gel by a process known as elution
D. DNA fragments separate according to their size through sieving effect of agarose gel

Answer

To solve the question regarding which statement is incorrect about gel electrophoresis, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Gel Electrophoresis**: Gel electrophoresis is a technique used to separate DNA fragments based on their size. The DNA is negatively charged and moves towards the positively charged anode when an electric field is applied.

2. **Analyzing the Options**:
- **Option 1**: "Agarose gel is used for separation of DNA fragments while polyacrylamide gel is used for protein separation."
- This statement is **correct**. Agarose gel is indeed used for DNA separation, while polyacrylamide gel is used for proteins.

- **Option 2**: "The separated DNA fragment can be visualized with or without staining with ethidium bromide."
- This statement is **incorrect**. Ethidium bromide staining is necessary to visualize the DNA fragments after separation. Without staining, the DNA bands cannot be seen.

- **Option 3**: "The desired band of the DNA containing gene of interest is extracted from the gel by a process known as elution."
- This statement is **correct**. The process of extracting the desired DNA band from the gel is indeed called elution.

- **Option 4**: "The DNA fragments separate according to their size through the sieving effect of the agarose gel."
- This statement is **correct**. The gel acts as a sieve, allowing smaller fragments to move faster than larger ones.

3. **Conclusion**: The only incorrect statement regarding gel electrophoresis is **Option 2**.

### Final Answer:
The incorrect statement regarding gel electrophoresis is: **"The separated DNA fragment can be visualized with or without staining with ethidium bromide."**