Which of the following cut DNA at specific sites ?

Options

A. Ligase
B. Exonuclease
C. Alkaline phosphatase
D. Restriction endonuclease

Answer

To solve the question "Which of the following cut DNA at specific sites?" we need to analyze each of the provided options:

1. **Ligase**: This enzyme is responsible for joining DNA fragments together. It acts like a molecular glue, facilitating the connection of DNA strands. Therefore, ligase does not cut DNA.

2. **Exonuclease**: Exonucleases are enzymes that remove nucleotides from the ends of DNA molecules. They can cleave nucleotides one at a time from either the 3' or 5' ends of the DNA strand. However, they do not cut DNA at specific internal sites; they only work at the ends.

3. **Alkaline Phosphatase**: This enzyme removes phosphate groups from the DNA backbone. While it modifies DNA, it does not cut the DNA itself.

4. **Restriction Endonuclease**: These enzymes are specifically designed to cut DNA at particular sequences. They recognize specific short nucleotide sequences and cleave the DNA at those sites, making them essential tools in molecular biology for DNA manipulation.

Based on this analysis, the correct answer to the question is **restriction endonuclease**, as it is the only option that cuts DNA at specific sites.

### Summary of Steps:
1. Identify the function of each enzyme listed in the options.
2. Determine whether each enzyme cuts DNA and if so, at what locations.
3. Conclude which enzyme is responsible for cutting DNA at specific sites.