#non-homologous end joining

Repair Pathway

Broken DNA ends in NHEJ are recognized by

Ku heterodimer composed of 70 kDa and 80 kDa subunits forms preformed ring with central aperture approximately 30 angstroms accommodating double-stranded DNA ends without sequence specificity. Abundant nuclear protein diffuses, threading onto broken terminus within seconds, protecting end from nucleolytic degradation and acting as scaffold for non-homologous end joining machinery. Ku recruits DNA-dependent protein kinase catalytic subunit, Artemis nuclease, polymerases mu and lambda, and XRCC4-Ligase IV complex. By synapsing two Ku-bound ends, it holds broken chromosome together. Rapid binding also antagonizes resection, steering repair away from homologous recombination.

Ref: Alberts Molecular Biology of Cell Chapter 5: Ku70/Ku80 end recognition in NHEJ; Nature Struct Mol Biol 2017 Ku-DNA complex

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Repair Pathway

NHEJ is predominant in which cell cycle phase?

Pathway choice for double-strand break repair depends strongly on cell cycle stage and resection control. In G0 quiescence and G1 gap where sister chromatids absent, extensive 5' resection blocked by 53BP1-RIF1-Shieldin complex and Ku end-binding factor prevents homology search. Therefore non-homologous end joining dominates, initiated by Ku70/Ku80 rapidly capping ends and recruiting DNA-PKcs and ligase IV. In S/G2, CDK phosphorylation activates CtIP enabling resection favoring homologous recombination. Thus NHEJ predominance in G0/G1 reflects availability of template and protective mechanisms limiting homologous recombination when homolog unavailable.

Ref: Berg Biochemistry Chapter 28: DSB repair cell cycle choice; NCBI Bookshelf NHEJ in G1 predominance regulation

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Recombination

Ku70–Ku80 complex is involved in

Mammalian double-strand break repair choice depends on cell cycle and end processing factors. Ku70-Ku80 heterodimer is initial sensor avidly threading onto free DNA ends with sequence-independent high affinity, physically blocking extensive resection by MRN-CtIP and Exo1. By recruiting DNA-PKcs, XRCC4, Ligase IV, Ku drives classical non-homologous end joining pathway which re-ligates ends with minimal processing generating small indels. Loss of Ku unmasks ends to homologous recombination or alternative microhomology-mediated end joining, shifting repair balance toward resection-dependent mechanisms particularly active in S phase, illustrating competitive pathway control.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 12: Ku Complex Involvement in Classical NHEJ

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Recombination

Non-homologous end joining (NHEJ) is

Non-homologous end joining ligates broken chromosome ends without requiring homologous donor template, acting throughout cell cycle especially G1. Core factors Ku70/Ku80 anchor DNA ends, DNA-PKcs protects, Artemis trims, polymerases mu/lambda add nucleotides, and Lig4-XRCC4 seals ligation. Since processing removes or adds bases before joining, small insertions or deletions often arise at junction, causing frameshifts and mutations. This error-prone nature distinguishes it from homologous recombination which copies intact template accurately, but affords rapid survival when sister chromatid unavailable, critical for V(D)J recombination and class switching.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: NHEJ as Error-Prone End Joining Mechanism

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