Structural Biochemistry/DNA ligase

DNA ligase is an enzyme that joins two DNA molecules or fragments together They are used in cloning where the recombination DNA molecules occur. The DNA ligase can also be used in DNA replication.

Genetic recombination happens through the breaking and reattachment of DNA molecules and not by replication where two molecules were copied through a DNA polymerase. Robert Lehman and his group determined that NAD was used as a cofactor for the bacteria cell, E.coli. A cofactor is something that is involved in enzyme-substrate catalysis and is usually either a metal or organometallic metal derived from vitamins. They found that NAD is confined to the bacteria while phages and mammalian ligases use ATP instead of NAD as their source of energy. Their experiments showed that DNA ligase is essential for joining Okazaki fragments during DNA replication and is also responsible for combining the DNA chains during nucleotide and base removal and repair of DNA. DNA ligase is also important for the joining of DNA fragments after the cleaving of a Holiday junction in recombination that is homologous. DNA ligase is now a key reagent in the construction of recombinant DNA molecules which leads to genetic variation.

Lehman, Robert. Wanderings of a DNA Enzymologist: From DNA Polymerase to Viral Latency. Annu. Rev. Biochem. January 16, 2006.