Figure 1.

A.

A. DNA is uncoiled at transcription “factories” within the cell, where the associated recombination and repair proteins co-localize.

B. The lymphoid specific recombinase activating gene 1 and 2 (RAG1/2) proteins recognize and bind the recombination signal sequences (RSS) that flank the V(D)J gene segments, and introduce site-specific DNA-DSBs.

C. The phosphorylated blunt signal ends and the covalently sealed hairpin intermediate of the coding end are held together by the RAG complex.

B.

D. The MRN complex binds the broken DNA ends and activates ATM which initiates cell cycle arrest and attraction of the repair proteins. H2AX, 53BP1 and RNF168, and with other proteins stabilize the damaged chromatin.

Ei. Ku70/Ku80 heterodimer binds the coding ends and recruits DNA-PKcs and Artemis, which is required to open the hairpin intermediates. The covalently sealed hairpin intermediate is randomly nicked by the DNA-Pkcs/Artemis complex, which generates a single stranded break with 3’ or 5’ overhangs. Eii. XRCC4, DNA ligase 4 and cernunnos-XLF (C-XLF) co-associate and are recruited to the ends. The signal ends are directly ligated by the XRCC4/DNALIG4/ C-XLF complex. The opened hairpin intermediate is modified by polymerases, exonucleases and the lymphoid-specific terminal deoxynucleotidyl transferase (TdT), before

Eiii. being repaired and ligated by the XRCC4/DNA-LIG4/C-XLF complex