# DNA Repair Mechanisms: How Damaged DNA Is Repaired

## Introduction to DNA Repair

DNA is the only macromolecule in the cell that is actively repaired rather than simply degraded and resynthesized. This distinction reflects the unique role of DNA as the repository of genetic information: a single unrepaired lesion can permanently alter the coding sequence, while even a transient replication error can become a fixed mutation in the next cell generation. The human genome sustains tens of thousands of damaging events per day from endogenous sources such as reactive oxygen species, hydrolysis, and alkylation, plus exogenous sources including ultraviolet (UV) radiation, ionizing radiation, and environmental chemicals. Without repair, the mutation rate would be orders of magnitude higher than the approximately 1 × 10⁻⁹ mutations per base pair per cell division actually observed.

DNA repair is not a single pathway but a coordinated network of systems, each specialized for particular classes of lesions. The major pathways are direct reversal, base excision repair (BER), [nucleotide excision repair](/knowledge/molecular-biology/nucleotide-excision-repair) (NER), mismatch repair (MMR), and the two double-strand break (DSB) repair pathways: non-homologous end joining (NHEJ) and [homologous recombination](/knowledge/molecular-biology/homologous-recombination) (HR). These pathways differ in the types of damage they recognize, the enzymes they employ, and the cell cycle stages at which they operate. Together they form the front line of genome maintenance, and their failure underlies numerous human diseases, including hereditary cancer syndromes such as Lynch syndrome and xeroderma pigmentosum.

### Types of DNA Damage

DNA damage falls into several broad categories based on the chemical nature of the lesion and the distortion it causes to the double helix.

**Base modifications** include oxidation products such as 8-oxoguanine (8-oxoG), which arises when reactive oxygen species attack guanine. 8-oxoG is mutagenic because it can pair with adenine as well as cytosine, leading to G:C → T:A transversions. Deamination converts cytosine to uracil, creating a U:G mispair that, if unrepaired, produces a C:G → T:A transition. Alkylation events add methyl or ethyl groups to bases; O⁶-methylguanine is particularly dangerous because it mispairs with thymine.

**Bulky adducts** are large chemical groups covalently attached to bases, such as the benzo[a]pyrene diol epoxide adducts from tobacco smoke or the cisplatin crosslinks used in chemotherapy. These lesions distort the DNA helix substantially and block both replication and transcription.

**UV-induced photoproducts** are a specific class of bulky lesions. Cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts form when adjacent pyrimidines absorb UV light and become covalently linked. CPDs are the most frequent UV lesion and are highly mutagenic, causing C → T transitions at dipyrimidine sites.

**Strand breaks** include single-strand breaks (SSBs), which can arise from oxidative damage to the sugar-phosphate backbone, and double-strand breaks (DSBs), which are the most cytotoxic lesion. DSBs can be induced by ionizing radiation, by replication forks encountering unrepaired SSBs, or by programmed processes such as V(D)J recombination in developing lymphocytes.

**Mismatches** are not chemical damage but replication errors: the incorporation of a non-complementary base by DNA polymerase. The error rate of replicative polymerases is approximately 10⁻⁵ to 10⁻⁶, and MMR reduces this to approximately 10⁻⁹.

### Consequences of Unrepaired Damage

Unrepaired DNA damage has three principal consequences. First, it can cause mutations: permanent changes in the DNA sequence that are passed to daughter cells. Second, it can trigger cell cycle arrest or apoptosis through the [DNA Damage Response](/knowledge/molecular-biology/dna-damage-response), a signaling network that coordinates repair with [cell cycle checkpoints](/knowledge/bioinformatics/cell-cycle-checkpoints-a-decision-framework-for-identifying-phase-specific-defects). Third, if damage blocks replication or transcription, it can cause [replication fork collapse](/knowledge/molecular-biology/replication-fork-collapse), generating secondary DSBs that are even more dangerous than the original lesion.

The clinical importance of DNA repair is underscored by inherited repair deficiencies. Mutations in NER genes cause xeroderma pigmentosum, characterized by extreme UV sensitivity and a >1000-fold increased risk of skin cancer. Defects in MMR genes cause Lynch syndrome, predisposing to colorectal and endometrial cancers. Mutations in BRCA1 or BRCA2, which function in HR, cause hereditary breast and ovarian cancer. These syndromes illustrate that DNA repair is not merely a cellular housekeeping function but a critical determinant of cancer susceptibility.

## Direct Reversal of DNA Damage

Direct reversal is the simplest repair strategy: a single enzyme recognizes a specific lesion and reverses it in a one-step reaction, without cutting the DNA backbone. This pathway is limited to a small number of lesion types, but it is highly efficient and energetically economical.

### Photoreactivation

Photoreactivation is the light-dependent repair of UV-induced pyrimidine dimers, catalyzed by the enzyme photolyase. Photolyase contains two chromophores: a catalytic flavin adenine dinucleotide (FADH⁻) and a light-harvesting antenna chromophore (either methenyltetrahydrofolate or 8-hydroxy-5-deazariboflavin, depending on the organism). The enzyme binds the CPD in the minor groove and flips the dimer out of the helix into its active site. Absorption of a photon in the blue-light range (350–450 nm) by the antenna chromophore transfers energy to FADH⁻, which donates an electron to the pyrimidine dimer. The resulting radical anion undergoes cycloreversion, breaking the cyclobutane ring and restoring the two intact pyrimidines. The electron is then transferred back to FADH⁻, regenerating the catalytic cofactor.

Photoreactivation is widespread in bacteria, fungi, plants, and many animals, but is absent in placental mammals, including humans. Humans instead rely exclusively on NER to remove UV photoproducts. This loss is evolutionarily significant: it means human cells cannot repair UV damage without going through the multi-step NER pathway, which is slower and requires more components.

### O6-Methylguanine Repair

O⁶-methylguanine (O⁶-meG) is a highly mutagenic lesion because it pairs with thymine rather than cytosine during replication, causing G:C → A:T transitions. The repair enzyme O⁶-methylguanine-DNA methyltransferase (MGMT, also called AGT in humans) transfers the methyl group from the O⁶ position of guanine to a cysteine residue in its own active site. This reaction is stoichiometric: one MGMT molecule repairs one lesion and is then irreversibly inactivated and targeted for ubiquitin-dependent degradation. MGMT is therefore a suicide enzyme, and the cell must synthesize new protein to restore repair capacity.

MGMT is constitutively expressed at low levels in most tissues but is induced by DNA damage. Its expression is clinically significant because MGMT removes the O⁶-methylguanine lesions produced by alkylating chemotherapeutics such as temozolomide. Tumors with high MGMT expression are resistant to these drugs, whereas tumors with silenced MGMT via promoter methylation (a common finding in glioblastoma) are more sensitive. This is a direct example of how understanding a DNA repair mechanism informs cancer treatment decisions.

## Base Excision Repair (BER)

BER is the primary pathway for repairing small, non-helix-distorting base lesions, including oxidized bases (8-oxoG), deaminated bases (uracil), alkylated bases (3-methyladenine), and abasic (AP) sites. BER is initiated by a family of enzymes called DNA glycosylases, each with specificity for particular lesions. The pathway is conceptually simple—remove the damaged base, cut the backbone, replace the nucleotide, and ligate—but it involves multiple subpathways and careful coordination.

### Recognition and Removal of Damaged Base

The first step of BER is catalyzed by a DNA glycosylase that recognizes the damaged base and cleaves the N-glycosidic bond between the base and the deoxyribose sugar, releasing the free base and leaving an apurinic/apyrimidinic (AP) site. There are two classes of glycosylases. Monofunctional glycosylases, such as uracil DNA glycosylase (UNG) and MutY homolog (MUTYH), possess only glycosylase activity. Bifunctional glycosylases, such as OGG1 (which removes 8-oxoG) and NTHL1 (which removes oxidized pyrimidines), also possess an associated AP lyase activity that nicks the DNA backbone 3′ to the AP site via a β-elimination reaction.

The mechanism of lesion recognition is remarkable. Glycosylases scan DNA by a process called facilitated diffusion, in which they bind non-specifically, slide along the helix, and interrogate each base pair. When a damaged base is encountered, the enzyme kinks the DNA and flips the base out of the helix into a deep active-site pocket that excludes normal bases by shape and hydrogen-bonding complementarity. This base-flipping mechanism ensures high specificity: UNG, for example, removes uracil from DNA with a catalytic efficiency that is 10⁶-fold higher than for any normal base.

After glycosylase action, the AP site is processed by AP endonuclease 1 (APE1 in humans). APE1 cleaves the phosphodiester backbone 5′ to the AP site, generating a single-strand break with a 3′-hydroxyl group and a 5′-deoxyribose phosphate (dRP) moiety. For bifunctional glycosylases that already nicked the 3′ side, APE1 removes the 3′-blocking group, generating the same 3′-OH/5′-dRP intermediate.

### Short-Patch and Long-Patch BER

BER proceeds through two subpathways that differ in the extent of repair synthesis. In **short-patch BER**, which handles the majority of lesions, DNA polymerase β (Pol β) inserts a single nucleotide complementary to the undamaged template strand. Pol β also possesses an intrinsic dRP lyase activity that removes the 5′-dRP moiety, leaving a nick with a 3′-OH and 5′-phosphate. The nick is sealed by DNA ligase IIIα in complex with XRCC1, a scaffold protein that coordinates the BER proteins.

In **long-patch BER**, used when the 5′-dRP is oxidized or otherwise resistant to Pol β's lyase activity, repair synthesis displaces the damaged strand to create a flap. Pol δ or Pol ε, together with the processivity factor PCNA, synthesizes 2–10 nucleotides, displacing the downstream strand. The flap is cleaved by flap endonuclease 1 (FEN1), and the nick is sealed by DNA ligase I. The choice between short-patch and long-patch BER depends on the lesion and the cell cycle stage, with long-patch BER predominating in proliferating cells where PCNA is available.

The [Base Excision Repair](/knowledge/molecular-biology/base-excision-repair) pathway is essential for genome stability: mice lacking core BER enzymes such as APE1 die during embryogenesis, and partial deficiencies in human glycosylases such as MUTYH cause a hereditary colorectal cancer syndrome (MUTYH-associated polyposis).

## [Nucleotide Excision Repair](/knowledge/molecular-biology/nucleotide-excision-repair) (NER)

NER removes bulky, helix-distorting lesions that BER cannot handle, including UV-induced photoproducts, bulky chemical adducts, and DNA crosslinks. Unlike BER, which removes only the damaged base, NER excises a short oligonucleotide (approximately 24–32 nucleotides in humans) containing the lesion, then fills the gap by repair synthesis. NER operates through two subpathways that differ only in damage recognition: global genomic NER (GG-NER), which surveys the entire genome, and transcription-coupled NER (TC-NER), which is triggered when RNA polymerase stalls at a lesion during transcription.

### Damage Recognition

In GG-NER, damage recognition is performed by the XPC-RAD23B complex, which detects the helical distortion caused by the lesion rather than the lesion itself. XPC binds to the undamaged strand opposite the lesion, and its affinity is enhanced by the UV-damaged DNA-binding protein (UV-DDB), a complex of DDB1 and DDB2 (the XPE gene product). UV-DDB recognizes UV photoproducts specifically and facilitates XPC loading. In TC-NER, the initiating event is a stalled RNA polymerase II at the lesion. The CSA and CSB proteins (mutated in Cockayne syndrome) recognize the stalled polymerase and recruit the same downstream NER machinery.

Following initial recognition, the TFIIH complex is recruited. TFIIH contains two helicases with opposite polarities: XPD (3′→5′) and XPB (5′→3′). These helicases unwind the DNA around the lesion, creating a bubble of approximately 20–30 nucleotides. XPD acts as a molecular ruler, verifying that a lesion is present by stalling when it encounters the damaged base. This verification step ensures that NER does not excise undamaged DNA.

### Incision and Excision

Once the DNA is unwound and the lesion is verified, two structure-specific endonucleases make incisions flanking the lesion. XPG cleaves the damaged strand 3′ to the lesion, and ERCC1-XPF cleaves 5′ to the lesion. The dual incision releases a 24–32 nucleotide single-stranded oligonucleotide containing the lesion. The exact length depends on the position of the lesion relative to the incision sites, which are determined by the boundaries of the open bubble.

The XPG and ERCC1-XPF endonucleases are both structure-specific: they recognize the junctions between single-stranded and double-stranded DNA at the edges of the bubble rather than the lesion itself. This ensures that the incisions are made at precise distances from the lesion, regardless of its chemical nature.

### Repair Synthesis and Ligation

The gap left by excision is filled by DNA polymerase δ or ε, working with PCNA and the replication factor RFC. The polymerase synthesizes DNA using the intact complementary strand as a template, and the nick is sealed by DNA ligase I or ligase IIIα-XRCC1. The entire process—from damage recognition to ligation—takes approximately 30 minutes in human cells.

The importance of NER is illustrated by the disease xeroderma pigmentosum, which results from mutations in any of seven genes (XPA through XPG). Patients with XP have a >1000-fold increased risk of skin cancer and often develop neurological degeneration due to accumulated DNA damage in neurons. Mutations in the TC-NER-specific genes CSA and CSB cause Cockayne syndrome, characterized by growth failure, neurological abnormalities, and premature aging but not increased skin cancer, reflecting the distinct roles of the two NER subpathways.

## Mismatch Repair (MMR)

MMR corrects replication errors that escape the proofreading activity of DNA polymerases. These errors include single-base mismatches (e.g., G:T or A:C) and small insertion-deletion loops (IDLs) that arise from strand slippage at repetitive sequences. MMR increases replication fidelity by 100–1000-fold and is essential for maintaining the stability of microsatellite sequences.

### Recognition of Mismatches

In bacteria, the MMR system is well characterized. The MutS protein recognizes the mismatch as a homodimer, and ATP binding induces a conformational change that allows MutS to recruit MutL. MutL activates MutH, an endonuclease that nicks the newly synthesized strand at a nearby hemi-methylated GATC site. The key to MMR is strand discrimination: the parental strand is methylated at GATC sequences by Dam methylase, while the newly synthesized strand is transiently unmethylated. MutH nicks only the unmethylated strand, ensuring that the correct (parental) sequence is preserved.

In humans, the system is more complex but follows the same logic. Mismatch recognition is performed by heterodimers of MutS homologs: MutSα (MSH2-MSH6) recognizes base mismatches and small IDLs, while MutSβ (MSH2-MSH3) recognizes larger IDLs. The MutL homologs form MutLα (MLH1-PMS2), which possesses a latent endonuclease activity. Importantly, humans lack a MutH homolog and do not use methylation for strand discrimination. Instead, strand discrimination is provided by the presence of pre-existing nicks in the newly synthesized strand, which occur naturally at Okazaki fragments on the lagging strand and at replication termination sites on the leading strand. The MMR machinery loads at these nicks and directs excision toward the mismatch.

### Excision and Resynthesis

After mismatch recognition and incision, the mismatched base and surrounding DNA are excised. The exonuclease EXO1 degrades the nicked strand from the nick toward the mismatch, working 5′→3′ or 3′→5′ depending on which side of the mismatch the nick lies. The single-stranded DNA generated by excision is protected by replication protein A (RPA). The gap is then filled by DNA polymerase δ, and the nick is sealed by DNA ligase I.

The importance of MMR is dramatically illustrated by Lynch syndrome (hereditary non-polyposis colorectal cancer), caused by germline mutations in MSH2, MLH1, MSH6, or PMS2. Affected individuals have a 70–80% lifetime risk of colorectal cancer and elevated risks of endometrial, ovarian, and other cancers. MMR-deficient tumors exhibit microsatellite instability (MSI), a hallmark phenotype in which repetitive sequences gain or lose repeat units due to unrepaired replication slippage. MSI testing is now a standard clinical tool for identifying Lynch syndrome patients and for predicting response to immune checkpoint inhibitors, since MMR-deficient tumors carry an exceptionally high mutation burden that makes them highly immunogenic.

The [Mismatch Repair](/knowledge/molecular-biology/mismatch-repair) pathway also plays a role in repairing mismatches that arise during [Semiconservative Replication](/knowledge/molecular-biology/semiconservative-replication), when the error rate of the replicative polymerases is highest.

## Double-Strand Break Repair: NHEJ and HR

Double-strand breaks (DSBs) are the most dangerous form of DNA damage. A single unrepaired DSB can cause cell death, and misrepair can generate chromosomal rearrangements that drive tumorigenesis. Cells have two principal DSB repair pathways: non-homologous end joining (NHEJ) and [homologous recombination](/knowledge/molecular-biology/homologous-recombination) (HR). These pathways differ fundamentally in their mechanism, their requirement for a homologous template, and their cell cycle dependence.

### Non-Homologous End Joining (NHEJ)

NHEJ directly ligates the two broken DNA ends without requiring sequence homology. It is active throughout the cell cycle but is the dominant DSB repair pathway in G1 phase, when no sister chromatid is available for HR. NHEJ is intrinsically error-prone because it often involves processing of the ends, which can delete or add nucleotides at the junction.

The pathway begins with the Ku70/Ku80 heterodimer, which binds with high affinity to DNA ends in a sequence-independent manner. Ku is a ring-shaped protein that threads onto the DNA end, and its binding protects the ends from degradation and recruits the other NHEJ factors. The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is recruited by Ku to form the DNA-PK holoenzyme. DNA-PKcs brings the two ends together in a synaptic complex and undergoes autophosphorylation, which induces conformational changes that allow access of processing enzymes.

If the ends are not directly ligatable, they must be processed. The Artemis nuclease opens hairpin structures and trims overhangs. Polymerases μ and λ fill in gaps in a template-dependent or template-independent manner. The XRCC4-DNA ligase IV complex, together with the accessory factor XLF, performs the final ligation. Ligase IV is specifically dedicated to NHEJ and can ligate ends that are not fully complementary, a property that contributes to the error-prone nature of the pathway.

NHEJ is essential for V(D)J recombination, the process that generates antibody and T-cell receptor diversity. During V(D)J recombination, the RAG1/RAG2 endonuclease creates programmed DSBs, which are joined by the NHEJ machinery. Humans with mutations in NHEJ genes such as ligase IV or Artemis suffer from severe combined immunodeficiency (SCID) due to failure of V(D)J recombination, in addition to radiation sensitivity.

### Homologous Recombination (HR)

HR repairs DSBs using an undamaged homologous sequence as a template, typically the sister chromatid. HR is therefore restricted to the S and G2 phases of the cell cycle, when a sister chromatid is available. HR is largely error-free because it copies the sequence from the intact template.

The first step of HR is resection: the 5′ ends of the break are nucleolytically degraded to generate 3′ single-stranded DNA (ssDNA) overhangs. Resection is initiated by the MRN complex (MRE11-RAD50-NBS1) together with CtIP, and is extended by the EXO1 exonuclease and the BLM helicase. The resulting RPA-coated ssDNA is then replaced by RAD51, a recombinase that forms a nucleoprotein filament on the ssDNA. This exchange is mediated by BRCA2, which loads RAD51 onto RPA-coated ssDNA and displaces RPA. The RAD51-ssDNA filament then searches for homologous sequences in the duplex DNA, a process that involves extensive sampling of the genome. When homology is found, the filament invades the homologous duplex, displacing one strand to form a displacement loop (D-loop).

The 3′ end of the invading strand is then extended by DNA polymerase using the homologous template. The subsequent steps depend on how the recombination intermediate is resolved. In the double Holliday junction pathway, the second end of the break is captured, and the junctions are resolved by structure-specific endonucleases (MUS81-EME1, GEN1, or SLX1-SLX4), generating crossover or non-crossover products. In the synthesis-dependent strand annealing (SDSA) pathway, the invading strand is displaced after synthesis and anneals to the other end, generating exclusively non-crossover products. SDSA is the predominant pathway in mitotic cells, where crossovers would cause loss of heterozygosity.

The [Homologous Recombination](/knowledge/molecular-biology/homologous-recombination) pathway is intimately connected to DNA replication. DSBs that arise from [replication fork collapse](/knowledge/molecular-biology/replication-fork-collapse) are repaired by HR, and the proteins involved in HR, including BRCA1, BRCA2, and RAD51, are essential for protecting stalled replication forks from nucleolytic degradation. This connection explains why BRCA1/2-mutant cancers are exquisitely sensitive to PARP inhibitors, which trap PARP1 on DNA and generate replication-associated DSBs that cannot be repaired in HR-deficient cells.

### Choice Between NHEJ and HR

The choice between NHEJ and HR is regulated by the cell cycle and by competition between the two pathways. The key determinant is resection: if resection occurs, the ends are committed to HR; if not, they are available for NHEJ. Resection is inhibited in G1 by the 53BP1 protein, which binds to damaged chromatin and blocks access of the resection machinery. In S/G2, BRCA1 counteracts 53BP1, allowing resection to proceed. The cyclin-dependent kinase (CDK) activity that rises in S phase also promotes resection by phosphorylating CtIP. This cell cycle regulation ensures that the error-free HR pathway is used only when a sister chromatid is available, and the error-prone NHEJ pathway is used when it is not.

The clinical relevance of DSB repair choice is illustrated by the use of PARP inhibitors in BRCA-mutant cancers. PARP1 is a DNA single-strand break sensor that also plays a role in replication fork stability. PARP inhibitors trap PARP1 on DNA, causing replication fork collapse and DSB formation. In HR-proficient cells, these DSBs are repaired accurately; in BRCA1/2-deficient cells, they cannot be repaired by HR and are instead processed by error-prone NHEJ, leading to genomic instability and cell death. This synthetic lethality approach is a paradigm for targeted cancer therapy.

For a more detailed comparison of the two pathways, see [Double Strand Break Repair](/knowledge/molecular-biology/double-strand-break-repair).

## Methods to Study DNA Repair

Studying DNA repair requires methods to induce damage, measure repair kinetics, and identify the proteins involved. Several techniques are standard in the field.

### Comet Assay

The comet assay (single-cell gel electrophoresis) measures DNA damage at the single-cell level. Cells are embedded in agarose on a microscope slide, lysed to remove membranes and proteins, and subjected to electrophoresis under alkaline or neutral conditions. Damaged DNA fragments migrate toward the anode, forming a "comet tail" behind the intact nucleoid "head." The extent of DNA in the tail is proportional to the number of strand breaks or alkali-labile sites. The comet assay is simple, quantitative, and requires only a few thousand cells. It is widely used to measure DNA damage induced by genotoxic agents and to assess the repair capacity of cells by measuring the disappearance of tails over time after damage.

### Immunofluorescence for Repair Proteins

Immunofluorescence (IF) allows visualization of DNA repair proteins at sites of damage. The most commonly used marker is γ-H2AX, the phosphorylated form of histone H2AX. Upon DSB formation, the kinase ATM phosphorylates H2AX at serine 139 over megabase-sized chromatin domains flanking the break. γ-H2AX foci can be detected with specific antibodies and counted by [fluorescence microscopy](/knowledge/diagnostics/imaging/fluorescence-microscopy-principles-applications-and-image-acquisition); each focus corresponds to one DSB. This assay is exquisitely sensitive and can detect single DSBs. Other repair proteins, such as RAD51 (HR), 53BP1 (NHEJ), and XPC (NER), can be visualized similarly, allowing researchers to determine which repair pathway is active at a given time. The kinetics of focus formation and disappearance provide a measure of repair efficiency.

### Reporter Constructs

Reporter assays measure the activity of specific repair pathways in living cells. The most widely used are the DR-GFP and EJ5-GFP systems for HR and NHEJ, respectively. In the DR-GFP assay, a GFP gene is inactivated by an I-SceI endonuclease cut site and a truncated GFP fragment is placed downstream. Expression of I-SceI creates a DSB in the inactive GFP; if the break is repaired by HR using the truncated fragment as a template, a functional GFP gene is restored, and the cell fluoresces green. The EJ5-GFP assay similarly measures NHEJ by restoring GFP expression after end joining. The fraction of GFP-positive cells, measured by flow cytometry, quantifies repair efficiency. These reporter systems can be integrated into cells and used to screen for genes that affect HR or NHEJ.

### CRISPR-Based Screens

CRISPR-Cas9 technology has revolutionized DNA repair research. Genome-wide knockout screens using CRISPR libraries allow identification of genes required for survival after DNA damage or for specific repair pathways. In a typical screen, cells are transduced with a lentiviral library of guide RNAs targeting all genes, treated with a DNA-damaging agent, and the guide RNAs that are depleted (indicating the gene is required for survival) are identified by sequencing. These screens have identified new repair factors and revealed genetic interactions that inform therapeutic strategies. CRISPR can also be used to introduce specific mutations into repair genes to study their function in isogenic cell lines.

## Common Pitfalls and Study Tips

Students learning DNA repair pathways often encounter several recurring difficulties. Addressing these directly will help you avoid common errors.

### Confusing BER and NER

BER and NER both remove damaged bases, but they handle different types of lesions and use different mechanisms. BER repairs small, non-helix-distorting lesions (oxidized, deaminated, alkylated bases) and removes only the damaged base, leaving an AP site that is processed by APE1. NER repairs bulky, helix-distorting lesions (UV photoproducts, bulky adducts) and excises a 24–32 nucleotide oligonucleotide. A useful mnemonic: BER = Base (small, single base), NER = Nucleotide (large, oligonucleotide). Remember that BER uses glycosylases and APE1, while NER uses XPC, TFIIH, XPG, and ERCC1-XPF. If you are asked which pathway repairs 8-oxoG, the answer is BER; if asked which repairs a CPD, the answer is NER.

### Overlooking Strand Discrimination in MMR

The critical feature of MMR is that it must repair the newly synthesized strand, not the template strand. In bacteria, this is achieved by methylation: the parental strand is methylated at GATC sites, and MutH nicks the unmethylated daughter strand. In humans, there is no methylation-based discrimination; instead, the repair machinery uses pre-existing nicks in the newly synthesized strand, which are abundant on the lagging strand at [Okazaki Fragment](/knowledge/molecular-biology/okazaki-fragment) junctions. If you are asked how MMR distinguishes the strands, do not answer "methylation" for humans. Also note that MMR does not repair chemical damage; it repairs replication errors (mismatches and IDLs).

### Remembering Key Proteins

The number of repair proteins can be overwhelming. Focus on the defining proteins for each pathway:

- **Direct reversal:** photolyase (photoreactivation), MGMT (O⁶-meG repair)
- **BER:** glycosylases (UNG, OGG1), APE1, Pol β, ligase IIIα-XRCC1
- **NER:** XPC-RAD23B (GG-NER), CSA/CSB (TC-NER), TFIIH (XPB, XPD), XPG, ERCC1-XPF
- **MMR:** MSH2-MSH6 (MutSα), MSH2-MSH3 (MutSβ), MLH1-PMS2 (MutLα), EXO1
- **NHEJ:** Ku70/Ku80, DNA-PKcs, Artemis, ligase IV-XRCC4
- **HR:** MRN (MRE11-RAD50-NBS1), CtIP, EXO1, BLM, RPA, RAD51, BRCA2

For each protein, ask yourself three questions: What does it recognize? What does it do? What happens if it is mutated? This framework will help you organize the information.

## Frequently Asked Questions

### How is DNA repaired?

DNA is repaired by multiple specialized pathways that recognize different types of damage. Direct reversal enzymes (photolyase, MGMT) chemically reverse specific lesions in a single step. Base excision repair removes small base lesions via glycosylases and AP endonucleases. Nucleotide excision repair excises bulky, helix-distorting lesions as part of a 24–32 nucleotide oligonucleotide. Mismatch repair corrects replication errors using strand discrimination to target the newly synthesized strand. Double-strand breaks are repaired by non-homologous end joining, which ligates the broken ends directly, or by homologous recombination, which uses a sister chromatid as a template for accurate repair.

### Can DNA be repaired?

Yes, DNA is continuously repaired by the cell. The human genome sustains tens of thousands of lesions per day, and the vast majority are repaired efficiently by the pathways described above. However, repair is not perfect: some lesions escape repair and become mutations, and some repair events are themselves error-prone (particularly NHEJ). The balance between damage and repair determines the mutation rate and the risk of cancer and aging-related degeneration.

### What happens if DNA damage is not repaired?

Unrepaired DNA damage has several consequences. It can cause mutations if the damage is present during replication and is bypassed by translesion synthesis polymerases, which often incorporate incorrect bases. It can block transcription, leading to cellular dysfunction. It can trigger the DNA damage response, activating cell cycle checkpoints that arrest the cell cycle to allow more time for repair. If damage is overwhelming or irreparable, the cell may undergo apoptosis (programmed cell death) or enter senescence. In multicellular organisms, the accumulation of unrepaired damage in stem cells can contribute to aging and cancer.

### What is the difference between base excision repair and nucleotide excision repair?

BER repairs small, non-helix-distorting lesions such as oxidized, deaminated, or alkylated bases. It removes only the damaged base, creating an AP site that is processed by APE1, and replaces one or a few nucleotides. NER repairs bulky, helix-distorting lesions such as UV photoproducts and large chemical adducts. It excises an oligonucleotide of 24–32 nucleotides containing the lesion and fills the gap by repair synthesis. BER is initiated by DNA glycosylases that flip the damaged base out of the helix; NER is initiated by damage recognition proteins (XPC in GG-NER, stalled RNA polymerase in TC-NER) that detect helical distortion.

### Why is mismatch repair important?

Mismatch repair corrects replication errors that escape polymerase proofreading, increasing replication fidelity by 100–1000-fold. Without MMR, the mutation rate rises dramatically, particularly at repetitive sequences where strand slippage causes insertion-deletion loops. MMR deficiency causes Lynch syndrome, a hereditary cancer predisposition syndrome, and MMR-deficient tumors exhibit microsatellite instability. MMR also plays a role in suppressing recombination between non-identical sequences and in signaling DNA damage.

### What are the two main pathways for repairing double-strand breaks?

The two main DSB repair pathways are non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ directly ligates the broken ends and is active throughout the cell cycle, but is error-prone because it often involves end processing that deletes or adds nucleotides. HR uses a homologous template (the sister chromatid) and is restricted to S and G2 phases, but is largely error-free. The choice between the pathways is regulated by cell cycle-dependent resection of the DNA ends.

### How do scientists measure DNA repair activity?

Scientists measure DNA repair activity using several approaches. The comet assay quantifies DNA strand breaks by electrophoresis of single cells. Immunofluorescence detects repair proteins or γ-H2AX foci at sites of damage, allowing visualization of repair foci and their kinetics. Reporter constructs such as DR-GFP (HR) and EJ5-GFP (NHEJ) measure pathway-specific repair efficiency by flow cytometry. CRISPR-based screens identify genes required for repair by genome-wide knockout followed by DNA damage survival assays. Biochemical assays can measure specific enzymatic activities, such as glycosylase cleavage or ligase activity, using defined DNA substrates.

## Key Takeaways

- DNA repair is a coordinated network of pathways, each specialized for particular lesion types: direct reversal, BER, NER, MMR, NHEJ, and HR.
- Direct reversal is the simplest mechanism, using single enzymes (photolyase, MGMT) to reverse specific lesions without cutting the DNA backbone.
- BER repairs small base lesions via glycosylases that flip the damaged base out of the helix, followed by APE1 cleavage, polymerase β insertion, and ligation.
- NER excises bulky, helix-distorting lesions as a 24–32 nucleotide oligonucleotide, using XPC for damage recognition in GG-NER and stalled RNA polymerase in TC-NER.
- MMR corrects replication errors and uses strand discrimination—methylation in bacteria, pre-existing nicks in humans—to ensure the newly synthesized strand is repaired.
- DSBs are repaired by NHEJ (error-prone, active throughout the cell cycle) or HR (error-free, restricted to S/G2), with the choice regulated by 53BP1 and BRCA1-dependent resection.
- Defects in DNA repair cause human diseases including xeroderma pigmentosum (NER), Lynch syndrome (MMR), and hereditary breast/ovarian cancer (HR), and repair pathways are direct targets for cancer therapy.

## Further Reading

- Ji WP, He NB. *Investigation on the DNA repaired gene polymorphisms and response to chemotherapy and overall survival of osteosarcoma*. International journal of clinical and experimental pathology. 2015. [PubMed 25755792](https://pubmed.ncbi.nlm.nih.gov/25755792/)
- Li C et al. *Exogenous melatonin enhances the tolerance of tiger nut (Cyperus esculentus L.) via DNA damage repair pathway under heavy metal stress (Cd(2+)) at the sprout stage*. Ecotoxicology and environmental safety. 2023. [PubMed 37769580](https://doi.org/10.1016/j.ecoenv.2023.115519)
- Pincheira J et al. *G2 checkpoint-dependent DNA repair and its response to catalase in Down syndrome and control lymphocyte cultures*. Cell biology international. 2007. [PubMed 17081779](https://doi.org/10.1016/j.cellbi.2006.09.018)
- Ley RD et al. *Excision repair characteristics of denV-transformed xeroderma pigmentosum cells*. Mutation research. 1989. [PubMed 2918865](https://doi.org/10.1016/0921-8777(89)90061-x)
- Ulmer KM, Gomez RF, Sinskey AJ. *Ionizing radiation damage to the folded chromosome of Escherichia coli K-12: repair of double-strand breaks in deoxyribonucleic acid*. Journal of bacteriology. 1979. [PubMed 374389](https://doi.org/10.1128/jb.138.2.486-491.1979)
- Xing J et al. *Versatile DNA Hydrogel-Mediated Delivery of Ginsenoside-Encapsulated Small Extracellular Vesicles to Boost Diabetic Wound Repair*. Advanced science (Weinheim, Baden-Wurttemberg, Germany). 2026. [PubMed 41538424](https://doi.org/10.1002/advs.202522920)

## Related Clinical & Scientific Guides

* [MAPK Pathway: Mechanism, Function, and Clinical Relevance](/knowledge/molecular-biology/mapk-pathway)
* [Mammalian Cell Culture Bioreactors: A Practical Guide](/knowledge/molecular-biology/mammalian-cell-culture-bioreactor)
* [Nucleotide Formation: Biosynthesis and Assembly of DNA/RNA Building Blocks](/knowledge/molecular-biology/nucleotide-formation)