What is the purpose of using DNA ligase in the construction of recomb…

The correct answer is Option (2) → To join the ends of cut DNA molecules

A restriction enzyme cuts the source DNA and the vector at specific places. The cut piece of DNA (for example, a gene) and the cut vector are then mixed, and DNA ligase is added. Ligase acts on the cut DNA molecules and joins their ends, so the gene gets sealed into the vector. The result is a new circular DNA made in vitro, the recombinant DNA.

Option (1) To cut DNA at specific locations — Incorrect. Cutting DNA at specific places is the job of restriction enzymes, the ‘molecular scissors’. Ligase joins; it does not cut.

Option (2) To join the ends of cut DNA molecules — Correct. Ligase seals the ends of the cut gene and the cut vector together.

Option (3) To deliver an alien piece of DNA into the host organism — Incorrect. The alien DNA is carried into the host by a vector, such as a plasmid. Ligase only builds the recombinant molecule; it does not deliver it.

Option (4) To multiply copies of the antibiotic resistance gene — Incorrect. Copies of a linked gene are made when the recombinant DNA replicates inside the host, using the vector’s origin of replication. Ligase does not make copies.

NCERT Extract:
Principles of Biotechnology – “… The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’– restriction enzymes. … In the same way, a plasmid can be used as vector to deliver an alien piece of DNA into the host organism. The linking of antibiotic resistance gene with the plasmid vector became possible with the enzyme DNA ligase, which acts on cut DNA molecules and joins their ends. This makes a new combination of circular autonomously replicating DNA created in vitro and is known as recombinant DNA.”