DNA Ligase

"In molecular biology, DNA ligase is a specific type of enzyme, a ligase, (EC 6.5.1.1) that repairs single-stranded discontinuities in double stranded DNA molecules, in simple words strands that have double-strand break (a break in both complementary strands of DNA). Purified DNA ligase is used in gene cloning to join DNA molecules together. The alternative, a single-strand break, is fixed by a different type of DNA ligase using the complementary strand as a template,^[1] but still requires DNA ligase to create the final phosphodiester bond to fully repair the DNA."

"DNA ligase has applications in both DNA repair and DNA replication (see Mammalian ligases). In addition, DNA ligase has extensive use in molecular biology laboratories for Genetic recombination experiments."

"The mechanism of DNA ligase is to form two covalent phosphodiester bonds between 3' hydroxyl ends of one nucleotide, ("acceptor") with the 5' phosphate end of another ("donor"). ATP is required for the ligase reaction, which proceeds in three steps: (1) adenylation (addition of AMP) of a residue in the active center of the enzyme, pyrophosphate is released; (2) transfer of the AMP to the 5' phosphate of the so-called donor, formation of a pyrophosphate bond; (3) formation of a phosphodiester bond between the 5' phosphate of the donor and the 3' hydroxyl of the acceptor."