Which of the following enzymes cut off nucleotides from 5' or 3' ends of DNA molecules?

Options

A. Endonucleases
B. Exonucleases
C. Restriction endonucleases
D. Alkaline phosphatases

Answer

To determine which enzymes cut off nucleotides from the 5' or 3' ends of DNA molecules, we can analyze the options provided:

1. **Endonucleases**: These enzymes cut DNA at specific internal sites within the DNA molecule. They do not remove nucleotides from the ends of the DNA strands. Therefore, endonucleases are not the correct answer.

2. **Exonucleases**: These enzymes are specifically designed to remove nucleotides from the ends of DNA molecules. They can cleave nucleotides from either the 5' end or the 3' end of the DNA strand. Hence, exonucleases are the enzymes that cut off nucleotides from the ends of DNA.

3. **Restriction Endonucleases**: These are a subset of endonucleases that cut DNA at specific sequences. They do not act on the ends of the DNA but rather on specific internal sites. Therefore, they are not the correct answer for this question.

4. **Alkaline Phosphatases**: These enzymes are involved in removing phosphate groups from nucleotides but do not cut nucleotides from the ends of DNA. Thus, they are also not the correct answer.

Based on this analysis, the correct answer to the question is **exonucleases**, as they are the enzymes that cut off nucleotides from the 5' or 3' ends of DNA molecules.

### Summary of Steps:
1. Identify the function of each enzyme option.
2. Determine which enzyme acts on the ends of DNA.
3. Conclude that exonucleases are the correct answer.