DNA Damage and Repair
Under scientific review
Educational content. Use it to learn concepts, not for individual clinical decisions.
DNA is a molecule subject to chemical alterations and breaks. Repair is not a single tool: different systems recognize different problems and restore DNA continuity and information where possible. Understanding this diversity helps distinguish damage, mutation and cellular responses.
Distinguish a lesion from a mutation; compare five repair pathways; interpret a damage marker without treating it as a diagnosis.
Before you begin: Cells and DNA and the cell cycle.
- Altered base
BER removes damaged bases; NER removes segments containing bulky lesions.
- Incorrect pairing
MMR corrects errors that escaped replication proofreading.
- Double-strand break
HR uses homology; NHEJ joins ends without a long template.
Damage is not synonymous with mutation
A lesion is a chemical or structural alteration: an oxidized base, an abnormal bond or a broken strand. A mutation is a sequence change. A lesion may be repaired before copying; if it interferes with replication and the error becomes fixed, a mutation may result. Not every lesion becomes a mutation, and not every mutation favors cancer.
Problems can arise inside cells through spontaneous chemical reactions, reactive oxygen species or replication errors. External agents, such as ultraviolet radiation, ionizing radiation and certain chemicals, can also damage DNA. Lesion type, location and cell state matter more than an isolated count.
A useful analogy distinguishes a torn page from a changed letter in a copy. The analogy ends when we remember that DNA is a dynamic molecule: the cell does not consult a perfect external archive. 2015 Chemistry Nobel: repair mechanisms.
Recognize, process and rebuild
- Recognize the problem: proteins detect structural changes, unsuitable bases or exposed ends.
- Prepare the site: repair may remove one base, excise a segment or process ends.
- Rebuild and join: DNA synthesis and ligation restore the molecule. The outcome depends on the mechanism and available substrate.
| Pathway | Full name | Typical problem | Strategy and participants |
|---|---|---|---|
| BER | Base excision repair | Small base lesions | A glycosylase starts base removal; other enzymes process and fill the site |
| NER | Nucleotide excision repair | Bulky lesions, including UV photoproducts | Excises a strand segment and resynthesizes using the other strand |
| MMR | Mismatch repair | Mismatches and small loops after replication | Complexes involving MSH2/MSH6 and MLH1/PMS2 participate in recognition and correction |
| HR | Homologous recombination | Some double-strand breaks | Uses homologous sequence; BRCA1, BRCA2 and RAD51 contribute at different steps |
| NHEJ | Non-homologous end joining | Double-strand breaks | Joins ends; Ku, DNA-PK and ligase IV participate |
BER, NER and MMR were characterized through biochemical studies of recognition and correction. Institutional overview. BRCA2 interaction with RAD51 has structural and functional support. Pellegrini et al., 2002.
Deeper dive: why does HR not fix every break?
HR frequently uses a sister chromatid, available after replication, and is therefore favored in S/G2. NHEJ operates across a broader part of the cycle and does not require that long template. NHEJ is not inevitably inaccurate: compatible ends can be joined precisely; end processing can change the sequence. Pathway selection involves competition, processing and context, not a conscious cellular choice. Research on end processing.
From damage response to experiment
The DNA damage response (DDR) connects recognition, signaling, repair and cell fate. ATM, ATR and DNA-PK participate in partially overlapping networks; the response can delay the cycle or favor senescence and death when problems persist.
Foci of γH2AX, a phosphorylated form of histone H2AX, are used to study responses associated with breaks and other stresses. They are chromatin signals, not photographs of individual breaks. Counting requires nuclear segmentation, staining controls, consistent exposure and attention to cell death. Rogakou et al., 1998.
Repair defects also help formulate therapeutic hypotheses. Experiments with BRCA1/2-deficient cells showed sensitivity to PARP inhibition; PARP participates in lesion responses. This illustrates synthetic lethality: combined vulnerabilities can be more harmful than either alone. Farmer et al., 2005. This does not mean that every BRCA variant implies the same function or response. NCI: BRCA variants.
Guided example
Conceptual experiment: a treatment increases γH2AX foci after one hour; they decrease after 24 hours.
- The observation is compatible with a transient damage response.
- Fewer foci alone do not prove complete repair: heavily damaged cells may have died or left the analysis.
- Compare viability, nuclear counts, untreated controls and a complementary integrity measure. The conclusion should match what was measured.
Check your understanding
Is a lesion already a mutation?
No. A chemical alteration or break may be repaired before a sequence change becomes fixed.
Are HR and NHEJ interchangeable?
No. Template availability, cell-cycle phase and end processing influence the pathway and outcome.
Do fewer foci prove DNA is back to normal?
No. Cell loss, technical differences and other explanations must be excluded.