DNA ligase is required for:

The correct answer is Option (2) → Joining of DNA fragments

DNA ligase is an enzyme that joins DNA pieces. It seals the ends of cut DNA molecules, so a gene of interest can be linked to a vector to form recombinant DNA. It also joins the discontinuous fragments that form on the lagging strand during replication.

(1) Cutting of DNA — Incorrect. Cutting is done by restriction endonucleases, which are the “molecular scissors”.

(2) Joining of DNA fragments — Correct. This is the work of DNA ligase.

(3) Degradation of DNA — Incorrect. Nucleases break DNA down. Ligase builds it up by joining.

(4) Synthesis of DNA — Incorrect. New DNA is made by DNA polymerase, which adds nucleotides to a growing strand. Ligase only joins pieces that already exist.

NCERT Extract:
Principles of Biotechnology – “… 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.”
The Machinery and the Enzymes – “… The discontinuously synthesised fragments are later joined by the enzyme DNA ligase (Figure 5.8).”