DNA Ligase

DNA ligase is the enzyme that finishes what a restriction enzyme starts: after two DNA ends (say, a vector and a foreign insert, both cut by the same restriction enzyme) have found each other and their complementary sticky ends have base-paired, there is still a physical break - a 'nick' - in the sugar-phosphate backbone on each strand. DNA ligase seals that nick permanently by forming a new phosphodiester bond between a free 5'-phosphate group on one side and a free 3'-hydroxyl group on the other, consuming ATP as the energy source for the reaction. Without this sealing step, the DNA would simply fall back apart the moment the weak, temporary base-pairing at the sticky ends came undone; ligase is what converts a loosely associated pair of DNA pieces into one continuous, stable, covalently joined molecule. The enzyme used for this …