DNA ligase
If a restriction enzyme is the molecular scissors that cut DNA, DNA ligase is the molecular glue that puts it back together. It is the enzyme that seals breaks in the DNA backbone, fusing two fragments into one continuous strand.
More precisely, it forms a phosphodiester bond between the loose end of one DNA strand and the start of another, closing the gap in the sugar-phosphate backbone. When two fragments have been brought together — for instance by complementary sticky ends that base-pair loosely — ligase makes that join permanent and covalent, so the molecule behaves as a single unbroken piece.
DNA ligase is not merely a laboratory tool; cells rely on it constantly. It seals the short fragments made on the lagging strand during DNA replication and helps repair breaks caused by damage. In genetic engineering, it is the indispensable partner of restriction enzymes: cut with one, join with the other, and you can build recombinant molecules to order.
After a gene and a cut plasmid base-pair through their sticky ends, DNA ligase seals the two nicks, welding them into a single recombinant circle.
Base-pairing holds the pieces; ligase makes the bond permanent.
Ligase joins blunt ends too, but the reaction is slower and less efficient than joining matching sticky ends, where the overhangs first hold the fragments in place by base-pairing.