Skip to content
Login Register

#double-strand breaks

4 public questions tagged with this topic.

Repair Pathway

Double-strand breaks are repaired error-free by

Double-strand break is most cytotoxic lesion resolved by two major pathways. Non-homologous end joining rapidly ligates ends but can delete nucleotides. Homology-directed repair, also termed homologous recombination repair or HDR, uses sister chromatid present in late S and G2 phases as faithful template. After resection by MRN complex CtIP, Rad51 catalyzes strand invasion forming D-loop, polymerase delta extends using homologous sequence, Holliday junctions resolved. Because homologous donor retains exact original sequence, repair restores information without errors, essential for maintaining genome stability and enabling precise genome editing applications using donor templates.

Ref: Lodish Molecular Cell Biology 9th ed. Chapter 12: HDR error-free DSB repair; Alberts Ch.5 Homologous recombination precise repair

View Details
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

View Details
Gene Tar getting

Cas9 endonuclease cuts the DNA and creates:

Cas9 is dual nuclease utilizing HNH domain to cut target strand complementary to guide RNA and RuvC domain to cut non-target strand simultaneously. Both nicks occur three base pairs upstream of PAM on opposite strands, generating predominantly blunt ended double strand breaks without single stranded overhangs. Such blunt breaks are predominantly repaired by non-homologous end joining causing small indels, or by homology directed repair if donor template provided. Blunt nature facilitates efficient ligation mediated insertion and predictable editing outcomes in genome engineering laboratories worldwide.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details