DNA ligase is an enzyme that anneals DNA strands together. It forms a phosphodiester bond between the complementary bases on two separate DNA strands. ATP is hydrolized in order to release a pyrophosphate, and the AMP that is formed attaches to the 5' end. The pyrophosphate bond activates the 5' end and a phosphodiester bond forms between the 5' phosphate and the 3' hydroxyl. DNA ligase is a crucial element in recombinant technology.
1. Alberts, Bruce. Molecular Biology of the Cell. 5th ed. New York: Garland Science, 2008, p 273.