DNA ligases catalyze formation of a phosphodiester bond between the 5' phosphate of one strand of DNA and the 3' hydroxyl of the another. This enzyme is used to covalently link or ligate fragments of DNA together. Most commonly, the reaction involves ligating a fragment of DNA into a plasmid vector, which is a fundamental technique in recombinant DNA work.
The most widely used DNA ligase is derived from the T4 bacteriophage. T4 DNA ligase requires ATP as a cofactor. A DNA ligase from E. coli is also available, but is not commonly used. The E. coli enzyme uses NAD as a cofactor.
Additional information on the use of T4 DNA ligase is presented in the section on DNA ligation.