DNA ligase is an enzyme that repairs irregularities or breaks in the backbone of double-stranded DNA molecules. It has three general functions: It seals repairs in the DNA, it seals recombination fragments, and it connects Okazaki fragments (small DNA fragments formed during the replication of double-stranded DNA). DNA ligase functions by forming a bond between the end of a “donor” nucleotide and the end of an “acceptor” nucleotide.
There are two main types of DNA ligase — the first is found only in prokaryotic cells (cells with no nucleus, such as bacteria). The second is found in eukaryotic cells (cells with a nucleus, like those of plants and animals) as well as in viruses and bacteriophages. Furthermore, mammals have four subtypes of ligases that vary in their function; DNA ligase III, for example, contains a DNA repair protein, called XRCC1, that seals the break in the DNA strand that occurs during nucleotide excision repair. In general, eukaryotic DNA ligases are much larger than their prokaryotic counterparts; the smallest DNA ligase is produced by the bacteriophage T7.
Because DNA ligase plays such an important role in assisting with DNA repair and replication, it is an important component of genetic recombination experiments, including cloning.