Non-homologous end joining (NHEJ) is a DNA repair pathway that reconnects the two ends of a double-strand break in DNA without requiring an extensive homologous template. Repair factors recognize, protect, align, and, when necessary, process the broken ends before joining them. In mammalian cells, classical or canonical NHEJ denotes the pathway organized around Ku and DNA ligase IV, distinguishing it from alternative end-joining mechanisms. NHEJ can restore the original sequence precisely or produce sequence changes, depending on the condition of the ends and the processing required. (nature.com)
Biological role and pathway choice
Double-strand breaks can result from ionizing radiation or programmed DNA cleavage, including reactions involved in immune-receptor assembly. NHEJ repairs both accidental and programmed breaks by joining available DNA ends rather than copying information from a separate DNA molecule. Its ability to handle structurally different ends makes it a flexible repair mechanism. However, reconnecting DNA and restoring its original sequence are not necessarily equivalent outcomes. (pubmed.ncbi.nlm.nih.gov)
NHEJ differs fundamentally from homologous recombination, which uses a homologous sequence as a repair template. Pathway choice is linked to the cell cycle and DNA-end resection—the degradation of the 5′ strands to expose single-stranded 3′ tails. Homologous recombination is favored when a replicated sister chromatid is available, whereas NHEJ is particularly important in G1 and remains active outside that phase. The 53BP1–RIF1 pathway counteracts extensive resection, while BRCA1–CtIP promotes conditions compatible with homologous repair. (pubmed.ncbi.nlm.nih.gov)
Molecular mechanism
End recognition and alignment. The Ku70–Ku80 complex binds exposed DNA ends. In mammalian cells, Ku recruits the catalytic subunit of DNA-dependent protein kinase, DNA-PKcs. Structural studies have identified a long-range complex containing Ku, DNA-PKcs, DNA ligase IV, XRCC4, and XLF, followed by a short-range complex in which the ends are aligned for processing and joining. These proteins therefore coordinate end protection and positioning as well as recruitment of catalytic factors. (nature.com)
End processing. Compatible ends may be joined with little or no modification. Other ends require removal of obstructing structures or synthesis across gaps. The Artemis nuclease, acting with DNA-PKcs, opens hairpin-shaped ends and processes certain overhangs. DNA polymerases can add nucleotides when synthesis is needed. Processing is coordinated with the joining machinery rather than occurring as an unrestricted degradation step; biochemical experiments show that compatible ends are protected from unnecessary modification. (sciencedirect.com)
Ligation. DNA ligase IV, associated with XRCC4 and supported by XLF, completes the joining reaction. Like other DNA ligases, it reconnects the DNA backbone by forming phosphodiester bonds. The XRCC4–ligase IV complex has unusual substrate flexibility: experiments show that it can join some incompatible ends and seal across small gaps under suitable conditions. Not every break requires an identical sequence of processing reactions before ligation. (nature.com)
Fidelity and alternative end joining
NHEJ is often described as error-prone, but it is not intrinsically inaccurate at every break. Direct ligation of suitable ends can preserve the original sequence. By contrast, trimming or nucleotide addition can produce mutations, particularly small insertions and deletions. In nuclease experiments, accurate repair may regenerate the recognition site and permit another cleavage event, so the final observed sequence changes can underrepresent earlier precise repair. (pubmed.ncbi.nlm.nih.gov)
Classical NHEJ can join ends without matching bases, but it can also use short matching sequences at a junction. Consequently, the presence of microhomology alone does not identify the repair pathway. Alternative mechanisms include microhomology-mediated end joining, in which resected ends are aligned through short sequence matches. DNA polymerase theta is a major factor in a prominent mammalian alternative pathway. These mechanisms are molecularly distinct from Ku–ligase IV-dependent repair and can generate deletions or chromosomal rearrangements. (pubmed.ncbi.nlm.nih.gov)
Immune-system functions and inherited defects
NHEJ is essential to V(D)J recombination, which assembles variable-region sequences during B-cell and T-cell development. Artemis processes hairpin coding ends, and the joining machinery reconnects the selected DNA segments. This process generates the genes encoding diverse antibodies and T-cell receptors. Classical NHEJ also contributes to class-switch recombination, although alternative end joining can support residual switching when classical components are absent. (sciencedirect.com)
Inherited defects demonstrate the connection between DNA repair and adaptive immunity. Pathogenic variants in LIG4, the gene encoding DNA ligase IV, have been identified in patients with immunodeficiency, developmental abnormalities, and cellular radiosensitivity. Clinical manifestations vary, and some disease-associated variants retain partial ligase activity rather than eliminating it completely. (pubmed.ncbi.nlm.nih.gov)
Bacterial systems and genome editing
NHEJ also occurs in some bacteria, but its distribution is uneven. Many bacterial systems use a comparatively compact machinery comprising a Ku homodimer and the multifunctional enzyme LigD. Depending on the organism, LigD combines ligation with polymerization and end-processing activities. Single-molecule experiments in Bacillus subtilis show that Ku can bridge DNA ends and that LigD stabilizes this interaction. (pmc.ncbi.nlm.nih.gov)
In genome editing, CRISPR nucleases such as Cas9 create breaks that cellular repair pathways resolve. End joining can generate insertions or deletions that disrupt a target gene, but outcomes depend on sequence context and repair machinery. Template-free repair products are not necessarily random, and not all should be attributed specifically to classical NHEJ. Experiments have also documented larger deletions and complex rearrangements after Cas9 cleavage, extending beyond the immediate cut site. (nature.com)