What is DNA ligase and what is its function?

DNA ligase is an enzyme that catalyzes the formation of phosphodiester bonds between the 3'-OH and 5'-phosphate ends of DNA strands, joining DNA fragments together. It is essential for DNA replication (joining Okazaki fragments), DNA repair, and recombinant DNA technology. T4 DNA ligase, derived from bacteriophage T4, is commonly used in molecular cloning to ligate DNA inserts into vectors. The enzyme requires ATP (or NAD+ in bacteria) as a cofactor for the ligation reaction.