Excision repair is the umbrella term for the cell’s main strategy of fixing damaged DNA: find the damage, cut it out, and fill the gap using the undamaged strand as a template. Discovered in 1964, this approach was the first evidence that cells actively maintain their genetic material rather than simply tolerating errors. Two major forms handle different kinds of damage: base excision repair deals with small, chemically altered individual bases, while nucleotide excision repair tackles larger, bulkier distortions in the double helix. When either system fails, the consequences range from accelerated aging to dramatically increased cancer risk, which is why excision repair has become a central target in modern cancer therapy.
How Base Excision Repair Works
Base excision repair, or BER, handles the most common type of DNA damage: individual bases that have been chemically modified by oxidation, deamination, or alkylation. These are subtle injuries. They do not dramatically warp the shape of the DNA helix, but they can cause the wrong base to be read during replication, leading to mutations. BER fixes tens of thousands of these lesions per cell per day, making it arguably the busiest repair pathway in the genome.
The process starts with a damage-specific enzyme called a DNA glycosylase. Each glycosylase recognizes a particular type of damaged base and snips it out, leaving behind what is known as an AP site (short for apurinic or apyrimidinic, meaning a spot where the base is simply missing). The human alkyladenine DNA glycosylase, for example, recognizes and removes alkylated bases. Its excision rate is deliberately slow, which researchers believe prevents AP sites from piling up faster than the next enzyme in line can handle them.1PubMed. Slow base excision by human alkyladenine DNA glycosylase limits the rate of formation of AP sites and AP endonuclease 1 does not stimulate base excision After the glycosylase acts, AP endonuclease 1 cuts the DNA backbone next to the empty site, and then a DNA polymerase fills in the correct base using the opposite strand as a guide. A ligase seals the nick, and the repair is complete.
BER operates through two sub-pathways. The short-patch pathway, which is the default route, replaces just one nucleotide. It relies primarily on DNA polymerase beta. The long-patch pathway replaces a stretch of two to six nucleotides and depends on a different replication factor called PCNA.2PubMed. Different DNA polymerases are involved in the short- and long-patch base excision repair in mammalian cells Which sub-pathway the cell chooses depends on the type of damage and the chemical nature of the intermediate left behind after the initial cut.3PubMed Central. DNA repair in mammalian cells: Base excision repair: the long and short of it
How Nucleotide Excision Repair Works
Nucleotide excision repair, or NER, deals with a different class of damage: bulky lesions that distort the physical shape of the DNA double helix. The classic example is the cyclobutane pyrimidine dimer caused by ultraviolet light, where two adjacent bases on the same strand become chemically fused together. Chemical carcinogens and certain chemotherapy drugs produce similar helix-warping adducts. NER is the pathway that handles all of them.
Instead of removing a single base, NER cuts out an entire short stretch of the damaged strand. Two structure-specific endonucleases make a pair of incisions flanking the lesion: XPG cuts on the 3′ side and the ERCC1-XPF complex cuts on the 5′ side.4PubMed Central. Mechanism of open complex and dual incision formation by human nucleotide excision repair factors Before these cuts are made, the cell uses energy from ATP to pry open a bubble of about 25 nucleotides around the damage site. The 3′ cut comes first, followed by the 5′ cut, and the damaged fragment (roughly 30 nucleotides long) is released as a single-stranded piece of DNA.5PubMed Central. Coordination of dual incision and repair synthesis in human nucleotide excision repair A polymerase fills the gap, and a ligase seals it shut.
NER is split into two sub-pathways based on how the damage is first detected. Global genome repair scans the entire genome for distortions and depends on the XPC protein complex to spot them. Transcription-coupled repair, by contrast, only finds damage on the strand of DNA that is actively being read into RNA; it is triggered when an RNA polymerase stalls at a lesion and relies on the CSB protein to signal for repair.6PLoS Genetics. Involvement of Global Genome Repair, Transcription Coupled Repair, and Chromatin Remodeling in UV DNA Damage Response Changes during Development Despite using different sensors, both sub-pathways feed into the same downstream machinery. Research has shown that XPC and the transcription-coupled repair factor UVSSA bind to nearly the same surface on a shared protein complex called TFIIH, meaning the two branches converge at a common recruitment step.7Nucleic Acids Research. Common TFIIH recruitment mechanism in global genome and transcription-coupled repair subpathways
Which Pathway Fixes What
A practical way to think about the division of labor is size and shape. BER handles subtle chemical changes to individual bases that do not seriously bend or distort the helix. NER handles bulky adducts and crosslinks that warp the DNA’s physical structure. UV-induced pyrimidine dimers are repaired by NER. Oxidized or alkylated individual bases, like the methylpurines produced by alkylating agents, are repaired by BER.8Oxford Academic. Repair of cyclobutane pyrimidine dimers or dimethylsulfate damage in DNA is identical in normal or telomerase-immortalized human skin fibroblasts
The division is not perfectly clean, though. Some oxidized bases, particularly those that introduce unusual ring structures, can be recognized by both pathways. In these cases, BER and NER proteins essentially compete for the same lesion, and which pathway wins depends on the local concentration of each set of recognition factors.9Genetics and Molecular Biology. Cooperation and interplay between base and nucleotide excision repair pathways: From DNA lesions to proteins This overlap matters because it means losing one pathway does not always leave a lesion completely unrepaired. The cell has built-in redundancy, at least for some types of damage.
What Happens When Excision Repair Fails
The clearest illustration of excision repair’s importance comes from inherited diseases where the pathway is broken. Xeroderma pigmentosum (XP) is caused by defects in NER proteins. People with XP are profoundly sensitive to sunlight because their cells cannot remove UV-induced DNA damage. Skin cancers that develop in XP patients carry a characteristic “UV signature” pattern of mutations, and the overall mutation burden in their tumors is strikingly high.10PubMed Central. Shining a light on xeroderma pigmentosum Genome sequencing of XP skin cancers has shown that certain subtypes carry mutation loads far above what is typical: tumors from XP-E patients, for example, averaged about 350 mutations per megabase of DNA, compared to roughly 130 in the broader dataset.11Nature Communications. Genomic mutation landscape of skin cancers from DNA repair-deficient xeroderma pigmentosum patients XP-C, one of the most common subtypes, results from a mutation in the XPC damage-recognition protein, which leaves global genome repair crippled while transcription-coupled repair continues to function.12PubMed Central. Synthetic rescue of Xeroderma Pigmentosum C phenotype via PIK3C3 downregulation
Cockayne syndrome is another NER-related disorder, but it presents very differently. Rather than cancer, its hallmarks are growth failure, neurological problems, and premature aging. The molecular basis has traditionally been linked to defects in transcription-coupled NER, though recent work suggests that problems in base excision repair and mitochondrial function also contribute.13PubMed Central. Cockayne syndrome: Clinical features, model systems and pathways The contrast between XP and Cockayne syndrome, both arising from faults in NER but manifesting so differently, hints at how much the specific protein affected and the sub-pathway disrupted determine the clinical outcome.
On the BER side, a clear disease link comes from MUTYH-associated polyposis, or MAP. The MUTYH glycosylase normally excises adenines that have been incorrectly paired with 8-oxoguanine, one of the most common products of oxidative DNA damage. When MUTYH is mutated, these mispairs persist and produce a characteristic pattern of G:C to T:A mutations.14DNA Repair. MUTYH-associated polyposis—From defect in base excision repair to clinical genetic testing This leads to the development of multiple colorectal adenomas, typically between 15 and 100, and a significantly increased risk of colorectal cancer.15PubMed Central. MUTYH the base excision repair gene family member associated with colorectal cancer polyposis
Excision Repair in Cancer Treatment
Excision repair is not just a topic for understanding disease; it is at the center of several modern cancer treatment strategies. The logic cuts two ways. Sometimes you want to block excision repair in tumor cells to make treatments more effective. Other times, a tumor’s existing repair deficiency creates a vulnerability you can exploit.
Cisplatin, one of the most widely used chemotherapy drugs, works by forming bulky adducts on DNA that kill rapidly dividing cells. But tumors frequently become resistant to cisplatin, and NER is a major reason why: the NER machinery recognizes and removes cisplatin-induced damage before it can trigger cell death.16PubMed Central. Role of Nucleotide Excision Repair in Cisplatin Resistance Researchers have responded by designing platinum-based prodrugs that simultaneously deliver DNA-damaging platinum and inhibit the NER pathway, aiming to prevent cancer cells from undoing the drug’s effects.17Angewandte Chemie. A Platinum(IV) Anticancer Prodrug Targeting Nucleotide Excision Repair To Overcome Cisplatin Resistance
PARP inhibitors represent the flip side of the coin. PARP-1 is an enzyme involved in detecting and signaling single-strand breaks, and it plays a role in BER. One model for how PARP inhibitors work proposes that blocking PARP-1 causes it to become trapped on DNA repair intermediates during base excision repair, creating obstructions that stall replication forks.18PubMed Central. The underlying mechanism for the PARP and BRCA synthetic lethality: clearing up the misunderstandings Cells with functioning homologous recombination can resolve these stalled forks, but tumor cells that carry BRCA1 or BRCA2 mutations cannot. The result is a “synthetic lethality” where neither the PARP inhibitor nor the BRCA mutation alone is lethal, but the combination is devastating to the cancer cell. Research has also shown that BER deficiencies themselves, such as loss of polymerase beta or the scaffolding protein XRCC1, can sensitize cells to PARP inhibitors even without a BRCA mutation, which raises the possibility of broadening who might benefit from these drugs.19PubMed Central. Base excision repair defects invoke hypersensitivity to PARP inhibition
The Chromatin Problem
In a test tube, excision repair enzymes work on naked DNA. Inside a living cell, DNA is wrapped tightly around histone proteins and packed into chromatin, which creates a physical barrier. Repair enzymes have to reach the damaged site, and that is harder when the damage happens to face inward toward the histone surface. Experiments have shown that shifting the position of a damaged base by just half a turn of the DNA helix on a histone surface can create a tenfold difference in how quickly BER enzymes can remove it.20PubMed Central. Accessing DNA damage in chromatin: Preparing the chromatin landscape for base excision repair
Cells have evolved several strategies to deal with this. Chemical modifications to histone tails can loosen the grip of chromatin. ATP-powered remodeling complexes physically slide or eject nucleosomes to expose damaged DNA. Swapping in variant histones can change the local chromatin structure.21PubMed Central. Regulation of DNA Repair Mechanisms: How the Chromatin Environment Regulates the DNA Damage Response These are not afterthoughts; they are essential parts of the repair process. Without chromatin remodeling, excision repair in living cells would be far slower and less complete than it is.
Excision Repair in Mitochondria
Mitochondria have their own small circular genome, and it faces a uniquely hostile environment. The mitochondrial DNA sits near the electron transport chain, where reactive oxygen species are constantly produced as a byproduct of energy generation. Unlike nuclear DNA, mitochondrial DNA is not wrapped in protective histone proteins, leaving it more exposed to oxidative damage.22PubMed Central. Base Excision Repair in the Mitochondria
BER is the primary repair pathway active in mitochondria. Several DNA glycosylases from the same families that work in the nucleus are also imported into mitochondria via targeting signals. Full NER, however, has long been thought to be absent or minimal in mitochondria. This means that bulky adducts that would be efficiently handled in the nucleus may persist in mitochondrial DNA, which has implications for aging and for how certain drugs affect mitochondrial function.
Alternative Excision Pathways
Beyond BER and NER, cells have a set of less-studied backup routes collectively called alternative excision repair. These pathways begin with a single nick introduced by a specialized endonuclease near the damage site, followed by excision, gap-filling, and ligation. The UV damage endonuclease found in fungi and bacteria, for instance, can initiate the repair of UV lesions independently of NER by cutting immediately next to the damage. Another enzyme, endonuclease V, recognizes deaminated bases and cleaves the DNA backbone one nucleotide away from the lesion, launching its own repair sequence.23PubMed Central. Alternative excision repair pathways These alternative routes are particularly interesting from an evolutionary standpoint because they offer organisms additional ways to cope with damage when the canonical pathways are overwhelmed or absent.
Conservation Across Life
One of the most striking things about excision repair is how ancient it is. NER exists in bacteria, archaea, and all eukaryotes. The bacterial version is remarkably streamlined: just three proteins (UvrA, UvrB, and UvrC) accomplish what takes 15 to 18 proteins in humans. Despite that enormous difference in complexity, the fundamental logic is the same in both systems. Both use a factor that recognizes helix distortion, a helicase that pries open the DNA around the lesion, and structure-specific endonucleases that make the cuts.24PubMed. Damage recognition in nucleotide excision repair of DNA That shared architecture across billions of years of divergent evolution underscores how essential the strategy of “cut it out and fill the gap” has been for the survival of DNA-based life.
Aging, Diet, and Individual Variation
Excision repair capacity is not fixed across a person’s lifetime or identical across individuals. There is evidence, albeit still fragmentary, that repair capacity declines as cells age. This decline has been observed in human cells approaching the end of their replicative lifespan in laboratory culture, and the pattern suggests that at least some types of repair become less efficient with age.25Gerontology. Relationship between DNA Repair Capacity and Cellular Aging Whether this contributes meaningfully to the accumulation of mutations in aging tissues is an active area of research.
Diet and sex also appear to matter. A study examining both genetic and dietary factors found that both BER and NER capacity were lower in women, and that higher blood levels of certain antioxidants, including ascorbic acid and alpha-carotene, were associated with higher NER capacity.26PubMed. Both genetic and dietary factors underlie individual differences in DNA damage levels and DNA repair capacity These are associations, not proof that eating more carrots will supercharge your DNA repair, but they point toward the idea that excision repair is not just a genetic hand you are dealt. It is a system whose performance is influenced by the body’s broader biochemical environment.
How Excision Repair Was Discovered
Before 1964, the prevailing assumption was that DNA was chemically stable and that cells simply tolerated whatever damage accumulated. The discovery of nucleotide excision repair upended that view by demonstrating that cells actively remove damaged stretches of DNA and accurately rebuild them using the undamaged complementary strand.27PubMed Central. Early days of DNA repair: discovery of nucleotide excision repair and homology-dependent recombinational repair This was, at the time, a genuinely surprising finding. It showed that the genome was not a static archive but an actively maintained document, and it opened the door to the entire modern field of DNA repair biology. The subsequent identification of BER, mismatch repair, and double-strand break repair pathways all built on the conceptual foundation that excision repair established: cells have evolved sophisticated, multi-step enzymatic systems to keep their genetic information intact.