*5.2.3 Rejoining of the broken ends of DNA*

For the completion of DNA repair process, the broken ends of the processed DNA must be rejoined. In NHEJ pathway, the rejoining and ligation step is carried out by Ligase IV, an ATP dependent enzyme. Ligase IV forms phosphodiester bonds between broken ends of DNA and catalyzing the ligation step. After hydrolysis of ATP, covalent linkage of AMP moiety occurs at specific lysine residue in the active site of DNA ligase. After linkage there is release of pyrophosphate [73]. This process releases AMP. Ligase IV has two C-terminal BRCT domains and is separated by a linker region. The linker region of Ligase IV interacts with the alpha helical region of XRCC4 to form an extremely stable complex. Till date, there are no published data on enzymatic activity of XRCC4 in DNA repairing process. XRCC4 is an important mediator for the recruitment of various NHEJ factors to the site of DNA damage and accelerate the process of DNA repair. Previous studies on NHEJ pathway in DNA repair process support the role of XRCC4 in stabilization and enhancement of DNA Ligase IV enzymatic activity [73–75]. DNA Ligase IV rejoins one strand of DNA at a time and simultaneously recruits and activates other repairing proteins that responsible for ligation of opposite strand of DNA.
