A foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using

Options

A. Eco R I
B. Taq polymerase
C. Polymerase III
D. Ligase

Answer

To answer the question, "A foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using," we need to analyze the options provided and identify which enzyme is responsible for joining the DNA fragments.

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases, like EcoRI, are enzymes that cut DNA at specific sequences. They create "sticky ends" that can facilitate the joining of different DNA fragments.
- **Hint**: Remember that restriction enzymes are used for cutting DNA, not for joining.

2. **Evaluating EcoRI**:
- EcoRI is a specific restriction endonuclease that recognizes the palindromic sequence 5' G A A T T C 3' and cuts the DNA at this site.
- Since EcoRI cuts DNA but does not join it, this option is incorrect.
- **Hint**: Focus on the function of EcoRI; it’s a cutter, not a joiner.

3. **Considering DNA Polymerases (Tag Polymerase and DNA Polymerase III)**:
- Both Tag polymerase and DNA Polymerase III are involved in DNA synthesis (polymerization). They add nucleotides to a growing DNA strand in the 5' to 3' direction.
- However, their role is not to join two separate DNA fragments together; they are used for DNA replication and synthesis.
- **Hint**: Think about the role of polymerases; they build DNA, not join fragments.

4. **Identifying the Role of Ligase**:
- Ligase is the enzyme responsible for joining two DNA fragments together. It creates phosphodiester bonds between the 3' hydroxyl end of one DNA fragment and the 5' phosphate end of another.
- This is the key function needed to form a recombinant plasmid from a foreign DNA fragment and a plasmid.
- **Hint**: Ligase is essential for DNA ligation; it’s the "glue" for DNA fragments.

5. **Conclusion**:
- The correct answer to the question is that a foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using **Ligase**.

### Final Answer:
The correct enzyme to join a foreign DNA and plasmid cut by the same restriction endonuclease is **Ligase**.