DNA ligase:

DNA ligases are specific type of enzymes that are involved in DNA repair mechanism in the cell. DNA ligases close the nicks in the DNA by forming phosphodiester bond between the 5'-phosphate of a nucleotide on one fragment of DNA and the 3'-hydroxyl of another. Inside the cell DNA ligases are mainly involved in DNA repair and in the joining of okazaki fragments formed during DNA replication. DNA ligases are mainly classified into two classes. The first class of ligases is found only in bacteria which use NAD + as a cofactor. The second class of ligase is mainly found in eukaryotes, viruses and bacteriophages. The second class of ligase use ATP as a cofactor. The most popularly used ligase in molecular biology studies is the smallest known ATP dependent DNA ligase (41 kDa) from bacteriophage T7. DNA ligases from most of the eukaryotes are much larger (>100 kDa) but all DNA ligase are known to share some common sequence and probably structural motif. T4 DNA ligase is known to ligate sticky ends of DNA more efficiently and but is less efficient in ligating blunt ends. Generally a high concentration of the enzyme is required in vitro reactions. To increase the reaction efficiency, the blunt ends are often modified using the enzyme terminal deoxynucleotidyl transferase which adds poly (dA) to DNA fragment from one source and poly (dT) to the DNA from another source. The complementary tails can form hydrogen bond and get ligated to generate a recombinant DNA.