Upon the action of the RAG 1/2 enzymes, the cleaved double-stranded DNA is left with hairpin structures at the end of each DNA segment created by the cleavage event. The hairpins are both opened by the Artemis complex, which has endonuclease activity when phosphorylated, providing the free 3' OH ends for TdT to act upon. Once the Artemis complex has done its job and added palindromic nucleotides (P-nucleotides) to the newly opened DNA hairpins, the stage is set for TdT to do its job. TdT is now able to come in and add N-nucleotides to the existing P-nucleotides in a 5' to 3' direction that polymerases are known to function. On average 2-5 random base pairs are added to each 3' end generated after the action of the Artemis complex. The number of bases added is enough for the two newly synthesized ssDNA segments to undergo microhomology alignment during non-homologous end joining according to the normal Watson-Crick base pairing patterns (A-T, C-G). From there unpaired nucleotides are excised by an exonuclease, like the Artemis Complex (which has exonuclease activity in addition to endonuclease activity), and then template-dependent polymerases can fill the gaps, finally creating the new coding joint with the action of ligase to combine the segments. Although TdT does not discriminate among the four base pairs when adding them to the N-nucleotide segments, it has shown a bias for guanine and cytosine base pairs.
