Practice question

Final sealing of processed DNA ends in classical non-homologous end joining performed by DNA ligase IV, dedicated ligase distinct from replication ligase I and base excision ligase III. Ligase IV forms stable obligate complex with XRCC4 scaffold and accessory factor XLF/Cernunnos plus PAXX, enhancing adenylation and end alignment. After Ku and DNA-PKcs orchestrate end processing, XRCC4-Ligase IV complex catalyzes ATP-dependent phosphodiester bond formation joining broken termini. Patient mutations in LIG4 cause immunodeficiency and radiosensitivity, highlighting essential role in lymphocyte V(D)J recombination and general double-strand break repair pathway completion.