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DNA ligase repairing chromosomal damage.
ligase I, DNA, ATP-dependent
Identifiers
Symbol	LIG1
Entrez	3978
HUGO	6598
OMIM	126391
RefSeq	NM_000234
UniProt	P18858
Other data
EC number	6.5.1.1
Locus	Chr. 19 [1]

ligase III, DNA, ATP-dependent
Identifiers
Symbol	LIG3
Entrez	3980
HUGO	6600
OMIM	600940
RefSeq	NM_002311
UniProt	P49916
Other data
Locus	Chr. 17 q11.2-q12

ligase IV, DNA, ATP-dependent
Identifiers
Symbol	LIG4
Entrez	3981
HUGO	6601
OMIM	601837
RefSeq	NM_002312
UniProt	P49917
Other data
Locus	Chr. 13 q33-q34

In molecular biology, DNA ligase is a particular type of ligase (EC 6.5.1.1) that can link together DNA strands that have double-strand breaks (a break in both complementary strands of DNA). The alternative, a single-strand break, is easily fixed by DNA polymerase using the complementary strand as a template 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 (see Applications in molecular biology research).

Contents 1 Ligase mechanism 2 Mammalian ligases 3 Applications in molecular biology research 4 See also 5 External links

The mechanism of DNA ligase is to form covalent phosphodiester bonds between 3' hydroxyl ends of one nucleotide with the 5' phosphate end of another. ATP is required for the ligase reaction.

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.

In mammals, there are four specific types of ligase.

• DNA ligase I: ligates Okazaki fragments during lagging strand DNA replication and some recombinant fragments.
• DNA ligase II: alternatively spliced form of DNA ligase III found in non-dividing cells.
• DNA ligase III: complexes with DNA repair protein XRCC1 to aid in sealing base excision mutations and recombinant fragments.
• DNA ligase IV: complexes with XRCC4. It catalyzes the final step in the non-homologous end joining DNA double-strand break repair pathway. It is also required for V(D)J recombination, the process which generates diversity in immunoglobulin and T-cell receptor loci during immune system development.

DNA ligases have become an indispensable tool in modern molecular biology research for generating recombinant DNA sequences. For example, DNA ligases are used with restriction enzymes to insert DNA fragments, often genes, into plasmids.

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 T_m (also the annealing temperature) of the DNA fragments being ligated.

If the ambient temperature exceeds T_m, homologous pairing of the sticky ends will not occur because the high temperature disrupts hydrogen bonding. The shorter the DNA fragments, the lower the T_m. 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).

The common commercially available DNA ligases were originally discovered in bacteriophage T4, E. coli or other bacteria.

• DNA end
• Lagging strand
• DNA replication
• Okazaki fragment
• DNA Polymerase

• DNA Ligation Protocol
• DNA Ligase: PDB molecule of the month
• Davidson College General Information on Ligase
• OpenWetWare DNA Ligation Protocol

v • d • e Enzymes: ligases (EC 6)
6.1 - Carbon-Oxygen	Aminoacyl tRNA synthetase
6.2 - Carbon-Sulfur	Succinyl coenzyme A synthetase - Acetyl Co-A synthetase - Long fatty acyl CoA synthetase
6.3 - Carbon-Nitrogen	Glutamine synthetase - Ubiquitin ligase (Von Hippel-Lindau tumor suppressor, UBE3A, Mdm2, Anaphase-promoting complex) - Glutathione synthetase - CTP synthase - Adenylosuccinate synthase - Argininosuccinate synthetase - Holocarboxylase synthetase - GMP synthase - Asparagine synthetase - Carbamoyl phosphate synthase (I , II )
6.4 - Carbon-Carbon	Carbon-carbon ligases
6.5 - Phosphoric Ester	DNA ligase

v • d • e DNA replication
Origin of replication/Ori/Replicon - DNA clamp - Okazaki fragment - Replication fork (Lagging and leading strands) - Single-strand binding protein - Primer - Processivity - Klenow fragment Pre-replication complex: Helicase (dnaA, dnaB, T7) - Primase (dnaG) - DNA polymerase III holoenzyme (dnaQ) DNA ligase - Telomerase - Topoisomerase