Why is DNA ligase needed after restriction enzymes?

Correct answer: To join cut DNA fragments

The governing concept is the complementary division of labour between restriction enzymes and DNA ligase in recombinant-DNA technology. A restriction enzyme cuts DNA at a particular recognition sequence and produces fragments, often with complementary sticky ends. Those ends may base-pair, but base pairing alone is not a permanent covalent connection. DNA ligase catalyses formation of phosphodiester bonds between adjacent nucleotides, sealing the breaks in the sugar-phosphate backbone and producing a continuous recombinant DNA molecule. Therefore option A is correct. DNA ligase does not colour DNA, digest fat, or generate light. Restriction digestion prepares the pieces, whereas ligation joins and stabilises them.