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 (see Applications in molecular biology research).
A pictorial example of how a ligase works (with sticky ends):
Ligase will also work with blunt ends, although higher enzyme concentrations and different reaction conditions are required.
One vital, and often tricky, aspect to performing successful recombination experiments involving ligase is controlling the optimal temperature. Most experiments use T4 DNA Ligase (isolated from bacteriophage T4) which is most active at 25°C. However in order to perform successful ligations, the optimal enzyme temperature needs to be balanced with the melting temperature Tm (also the annealing temperature) of the DNA fragments being ligated.
If the ambient temperature exceeds Tm, homologous pairing of the sticky ends will not occur because the high temperature disrupts hydrogen bonding. The shorter the DNA fragments, the lower the Tm. Thus for sticky ends (overlaps) less than ten base pairs long, ligation experiments are performed at very low temperatures (~4-8°C) for a long period of time (often overnight).