DNA fragments can be joined together by:

The correct answer is Option (2) → DNA ligase

(1) RNA polymerase — Incorrect. It makes RNA from a DNA template during transcription. It does not join pieces of DNA.

(2) DNA ligase — Correct. DNA ligase acts on cut DNA molecules and joins their ends. This is how a foreign gene is sealed into a plasmid vector to make recombinant DNA. The same enzyme joins the Okazaki fragments during DNA replication.

(3) Endonuclease — Incorrect. An endonuclease cuts DNA at specific points inside the molecule. Restriction endonucleases are the ‘molecular scissors’ of genetic engineering. They cut; they do not join.

(4) Exonuclease — Incorrect. An exonuclease removes nucleotides from the ends of DNA. It also cuts; it does not join.

Note: A similar question was also asked in CUET (UG) 2026 on 19-05-2026 (Slot 2).

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. The cut piece of DNA was then linked with the plasmid DNA. These plasmid DNA act as vectors to transfer the piece of DNA attached to it. You probably know that mosquito acts as an insect vector to transfer the malarial parasite into human body. 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.”