If the plasmid and the foreign DNA are cut by the same restriction endonuclease, recombinant DNA can be formed by joining both by

The correct answer is Ligase.

Step-by-Step Explanation:

1. Understanding the Role of Restriction Endonucleases:
When a plasmid (vector DNA) and a foreign DNA fragment are cut by the same restriction endonuclease, they generate complementary DNA ends. These ends can either be "sticky" (having single-stranded overhanging sequences) or "blunt" (having no overhangs). Because the same enzyme is used for both, the overhangs on the plasmid and the foreign DNA will be perfectly complementary to each other, allowing them to pair up via hydrogen bonding.

2. The Need for a Joining Enzyme:
While hydrogen bonding can temporarily hold the complementary overhangs together, it does not create a continuous, covalently bonded DNA molecule. The sugar-phosphate backbone of the DNA strands remains broken, leaving nicks where adjacent nucleotides are not covalently linked.

3. Role of DNA Ligase:
To seal these nicks and form a stable recombinant DNA molecule, an enzyme called DNA ligase is required. DNA ligase catalyzes the formation of a covalent phosphodiester bond between the 3'-hydroxyl (3'-OH) end of one DNA strand and the 5'-phosphate (5'-PO₄^3-) end of another. This structural seal permanently joins the foreign DNA insert into the plasmid vector.

4. Evaluating Other Options:
• Polymerase III: This enzyme is primarily involved in DNA replication (synthesizing new DNA strands by adding nucleotides), not in joining two pre-existing double-stranded DNA molecules together.
• EcoRI: This is a specific restriction endonuclease used to cut DNA at a specific recognition sequence (5'-GAATTC-3'), not to join DNA fragments.
• Taq polymerase: This is a thermostable DNA polymerase used in Polymerase Chain Reaction (PCR) to amplify DNA sequences at high temperatures; it does not join plasmid and foreign DNA fragments together.