DNA ligases catalyze formation of a phosphodiester bond between the 5' phosphate (5'P) of one strand of DNA and the 3' hydroxyl (3'OH) of the another.
DNA ligases close nicks in the phosphodiester backbone of DNA and are essential for the joining of Okazaki fragments during replication, and for completing short-patch DNA synthesis occurring in DNA repair process.
There are two classes of DNA ligases.
The first uses NAD+ as a cofactor and only found in bacteria.
The second uses ATP as a cofactor and found in eukaryotes, viruses and bacteriophages.