GO:0006297 nucleotide-excision repair, DNA gap filling: Mechanism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006297 describes the final DNA synthesis step of nucleotide excision repair (NER), in which DNA polymerase fills the single-stranded gap left after the damaged segment is removed.
• Gap filling is not a passive 'fill-in' reaction: it requires the coordinated action of DNA polymerases, replication factor C (RFC), proliferating cell nuclear antigen (PCNA), and DNA ligase I to restore an intact duplex.
• Defects in NER gap filling are linked to cancer predisposition, accelerated aging, and, in senescent cells, double-strand break formation.
• The process is regulated by ubiquitin-mediated degradation of XPG, which promotes the hand-off from incision to gap-filling DNA synthesis.
• Experimental dissection of gap filling uses knockout, point-mutation, knock-in, and overexpression cell models combined with DNA repair assays, sequencing, and imaging.
• Understanding GO:0006297 is essential for interpreting how cells maintain genome stability and for developing therapies that target DNA repair vulnerabilities.
Description
Nucleotide excision repair (NER) is the major pathway that removes bulky, helix-distorting DNA lesions such as ultraviolet-induced cyclobutane pyrimidine dimers and 6-4 photoproducts. The NER reaction can be divided into damage recognition, dual incision, damage removal, gap filling, and ligation. GO:0006297, nucleotide-excision repair, DNA gap filling, specifically covers the step in which the single-stranded gap generated by excision of the damaged oligonucleotide is filled by DNA polymerase and sealed by DNA ligase. This step is essential because an unrepaired gap would persist as a strand break, threatening replication, transcription, and chromosome integrity. For researchers, GO:0006297 provides a precise functional annotation for genes and proteins that act after the incision stage. It distinguishes gap-filling factors from lesion-recognition or incision factors, enabling more accurate interpretation of CRISPR screens, transcriptomic data, and DNA repair assays. Because gap filling is the committed step that restores the genetic information, its efficiency and fidelity directly influence mutation load, cellular senescence, and cancer risk. This article summarizes the authoritative QuickGO definition of GO:0006297, the molecular players involved, the experimental models used to study it, and the human diseases linked to its dysfunction. All statements are based on published literature cited by PMID.
nucleotide-excision repair, DNA gap filling At A Glance
| GO ID | GO:0006297 |
|---|---|
| GO term | nucleotide-excision repair, DNA gap filling |
| Ontology | biological_process |
| Synonym | none |
| Major function | Fills the single-stranded DNA gap generated during nucleotide excision repair using DNA polymerase and DNA ligase |
| Parent process | nucleotide-excision repair (GO:0006289) |
| Cellular location | nucleus |
| Key enzymes | DNA polymerase delta, DNA polymerase epsilon, DNA polymerase kappa, DNA ligase I, DNA ligase III |
| Accessory factors | PCNA, RFC, XPG, Cdt2 |
| Disease relevance | Cancer predisposition, premature aging, senescence-associated DNA damage |
What Is GO:0006297?
GO:0006297 (nucleotide-excision repair, DNA gap filling) is defined as the repair of the gap in the DNA helix by DNA polymerase and DNA ligase after the portion of the strand containing the lesion has been removed by pyrimidine-dimer repair enzymes. In other words, once NER incision factors have cut out the damaged DNA segment, this term describes the subsequent DNA synthesis that copies the intact template strand and the ligation step that seals the newly synthesized patch into the chromosome.
Why Is nucleotide-excision repair, DNA gap filling Important in Cell Biology?
Gap filling is the step that converts an excised, potentially mutagenic lesion into a restored DNA duplex. Without efficient gap filling, NER would leave single-strand breaks that can collapse replication forks, trigger double-strand breaks, and activate DNA damage responses. Because this step determines whether repair is complete and error-free, it is central to genome stability, normal development, and prevention of cancer and age-related pathologies.
• Restores the correct DNA sequence after removal of UV-induced pyrimidine dimers and other bulky adducts.
• Prevents persistent single-strand breaks that can be converted to double-strand breaks in senescent cells.
• Maintains genome stability and reduces mutation load, thereby lowering cancer risk.
• Coordinates with base excision repair (BER) gap-filling machinery, sharing polymerases and ligases.
• Is regulated by ubiquitin-mediated degradation of XPG, which couples incision to gap-filling synthesis.
• Provides a functional annotation for interpreting CRISPR screens that identify DNA repair dependencies.
• Is relevant to chemoresistance, as cancer cells may upregulate gap-filling capacity.
• Serves as a target for experimental models of premature aging and neurodegeneration linked to DNA repair defects.
• Enables accurate interpretation of transcriptomic and proteomic data in DNA damage response studies.
• Underpins the development of precision therapies that exploit DNA repair deficiencies.
What Happens During nucleotide-excision repair, DNA gap filling?
Recognition and incision create the gap
In simple terms: First, the cell finds and cuts out the damaged piece of DNA, leaving a hole.
NER begins with damage recognition by XPC-RAD23B or the transcription-coupled CSA/CSB complex, followed by TFIIH recruitment and verification of the lesion. Dual incision by XPF-ERCC1 and XPG on either side of the damage releases a 24-32 nucleotide oligonucleotide, creating a single-stranded gap in the DNA helix.
Gap-filling DNA synthesis
In simple terms: A DNA polymerase uses the undamaged strand as a template to copy the missing sequence.
After incision, the gap is filled by DNA polymerase delta or epsilon in replicating cells, or by DNA polymerase kappa in some contexts. Proliferating cell nuclear antigen (PCNA) is loaded onto DNA by replication factor C (RFC) and acts as a sliding clamp that tethers the polymerase to the primer-template junction, enhancing processivity.
Ligation seals the patch
In simple terms: The final nick is sealed by a DNA ligase to make the DNA continuous again.
Once the gap is filled, DNA ligase I (or DNA ligase III in some contexts) catalyzes phosphodiester bond formation between the 3'-hydroxyl end of the newly synthesized patch and the 5'-phosphate of the downstream DNA, restoring an intact strand.
Coordination with other repair pathways
In simple terms: The gap-filling step shares tools with other DNA repair systems.
The gap-filling machinery overlaps with base excision repair (BER), where DNA polymerase beta and DNA ligase III perform similar roles. This shared usage suggests that cells coordinate gap filling across pathways to maintain genome integrity.
Regulation by XPG degradation
In simple terms: The incision protein XPG is destroyed after it does its job, which helps the gap-filling step proceed.
Cdt2-mediated ubiquitination and degradation of XPG after incision promotes gap-filling DNA synthesis, ensuring that the repair intermediate is efficiently converted to a sealed product. This regulation links the incision and gap-filling stages of NER.
Key Genes Involved in GO:0006297 nucleotide-excision repair, DNA gap filling
The following genes and proteins are experimentally implicated in the gap-filling step of nucleotide excision repair (GO:0006297) or in closely related gap-filling processes.
| Gene | Major Role | Research Relevance |
|---|---|---|
| XPA | Damage verification and recruitment of repair factors | Defects cause xeroderma pigmentosum; used in NER assays |
| XPC | Initial damage recognition in global genome NER | Knockout models show impaired gap filling |
| ERCC1 | 5' incision during NER | Deficiency leads to repair defects and cancer predisposition |
| XPF | 5' incision partner of ERCC1 | Mutations cause xeroderma pigmentosum and Cockayne syndrome |
| XPG | 3' incision; its degradation promotes gap filling | Regulated by Cdt2; knockout affects gap-filling efficiency |
| PCNA | Sliding clamp for DNA polymerases | Essential for processive gap filling; target for inhibition studies |
| RFC | Loads PCNA onto DNA | Required for gap-filling synthesis in vitro |
| POLA1 | DNA polymerase alpha; potential backup gap filler | Studied in context of replication-coupled repair |
| POLD1 | DNA polymerase delta; main gap-filling polymerase | Knockout is lethal; point mutants affect repair |
| POLE | DNA polymerase epsilon; gap filling in some contexts | Mutations linked to colorectal cancer |
| POLK | DNA polymerase kappa; translesion synthesis and gap filling | Overexpression models show altered repair |
| LIG1 | DNA ligase I; seals the final nick | Defects cause immunodeficiency and growth retardation |
| LIG3 | DNA ligase III; backup ligase in some repair pathways | Studied in BER and NER cross-talk |
| POLB | DNA polymerase beta; gap filling in BER | Shares mechanism with NER gap filling |
| Cdt2 | E3 ubiquitin ligase that targets XPG for degradation | Regulates the incision-to-gap-filling transition |
| DDB1 | Part of CRL4 ubiquitin ligase complex with Cdt2 | Involved in XPG degradation |
| CUL4A | Scaffold for CRL4 ubiquitin ligase | Regulates NER gap filling via XPG |
| TET2 | DNA demethylation enzyme; influences DNA damage response | Links active demethylation to gap filling |
How Is nucleotide-excision repair, DNA gap filling Regulated?
The transition from incision to gap filling in NER is regulated by ubiquitin-mediated proteolysis. Cdt2, in complex with DDB1 and CUL4A, recognizes XPG after it performs incision and targets it for degradation, thereby promoting gap-filling DNA synthesis. This regulatory mechanism ensures that the repair intermediate is not left as a persistent nick and that the gap-filling machinery can access the DNA. Additionally, active DNA demethylation mediated by TET enzymes has been linked to the DNA damage response, suggesting that epigenetic modifications can influence gap-filling efficiency. The process is also coordinated with cell cycle progression, as PCNA and RFC are cell-cycle regulated.
nucleotide-excision repair, DNA gap filling and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| XPA | Xeroderma pigmentosum | Knockout keratinocytes or fibroblasts |
| XPG | Xeroderma pigmentosum, Cockayne syndrome | Point mutation knock-in in HEK293T |
| ERCC1 | Xeroderma pigmentosum, premature aging | Liver-specific knockout mouse |
| POLE | Colorectal cancer with ultramutation | Knock-in of exonuclease domain mutations |
| LIG1 | Immunodeficiency, growth retardation | Conditional knockout in B cells |
Cancer predisposition and NER gap filling
Defects in nucleotide excision repair, including gap-filling steps, are associated with increased cancer risk. Mutations in NER genes such as XPA, XPC, ERCC1, and XPF cause xeroderma pigmentosum, a condition characterized by extreme sensitivity to UV light and a high incidence of skin cancers. The gap-filling polymerases and ligases are also implicated in cancer: for example, POLE mutations are found in colorectal cancers with ultramutator phenotypes. Understanding gap filling may reveal therapeutic vulnerabilities in tumors with defective NER.
Senescence and double-strand break formation
Faulty gap filling during NER can lead to the formation of double-strand breaks in senescent cells. Suzuki et al. (2025) demonstrated that inefficient gap filling in NER results in persistent single-strand breaks that are converted to double-strand breaks, contributing to the senescence-associated secretory phenotype and age-related pathologies. This links GO:0006297 directly to cellular aging and degenerative diseases.
Neurodegeneration and premature aging
Impaired DNA repair, including gap filling, is a hallmark of premature aging disorders such as Cockayne syndrome and xeroderma pigmentosum neurological variants. These conditions feature progressive neurodegeneration, growth failure, and photosensitivity. The accumulation of unrepaired DNA lesions and repair intermediates can trigger apoptosis in post-mitotic neurons, highlighting the importance of efficient gap filling for neuronal survival.
From nucleotide-excision repair, DNA gap filling-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of XPG affect gap-filling efficiency? | XPG knockout cell line (e.g., HeLa or HEK293T) |
| Does a cancer-associated POLE mutation impair gap filling? | POLE point-mutation knock-in cell line |
| Can a tagged polymerase be used to monitor gap filling in live cells? | Knock-in of fluorescent tag (e.g., GFP) at POLA1 or POLD1 locus |
| Does overexpression of POLK enhance gap filling? | Doxycycline-inducible POLK overexpression in fibroblasts |
| Does Cdt2-mediated XPG degradation require a specific degron? | Point mutations in XPG degron via CRISPR |
| Does TET2 loss alter gap-filling kinetics? | TET2 knockout embryonic stem cells |
How to Study the nucleotide-excision repair, DNA gap filling Process
| Method | What It Measures | Typical Application |
|---|---|---|
| In vitro gap-filling assay | Incorporation of labeled nucleotides into a gapped DNA substrate | Measuring enzymatic activity of polymerases |
| Comet assay | DNA strand breaks at single-cell level | Detecting persistent gaps in repair-deficient cells |
| XR-seq | Genome-wide mapping of excised oligonucleotides | Profiling NER activity and gap filling |
| RNA-seq | Transcriptional changes after DNA damage | Identifying gap-filling gene expression signatures |
| Proteomics (IP-MS) | Protein interactions with repair intermediates | Discovering novel gap-filling factors |
| Live-cell imaging | Recruitment kinetics of fluorescently tagged proteins | Visualizing gap-filling factor dynamics |
| CRISPR screen | Gene essentiality or sensitivity to DNA-damaging agents | Identifying gap-filling dependencies |
| Whole-genome sequencing | Mutational signatures and repair fidelity | Linking gap-filling defects to cancer |
DNA repair assays
In vitro gap-filling assays using cell-free extracts or purified proteins measure the incorporation of radiolabeled nucleotides into a defined DNA substrate containing a single-stranded gap. These assays can be coupled with UV irradiation or defined lesions to specifically monitor NER gap filling. Comet assays and alkaline unwinding can detect single-strand breaks that persist when gap filling is defective.
Sequencing-based approaches
Next-generation sequencing of repair intermediates, such as XR-seq (excision repair sequencing), can map excised oligonucleotides and infer gap-filling activity. RNA-seq and ribosome profiling can reveal transcriptional and translational responses to DNA damage, including upregulation of gap-filling genes. Whole-genome sequencing of cells with defective gap filling can identify mutational signatures associated with repair deficiency.
Proteomics and interactomics
Mass spectrometry-based proteomics can identify proteins that associate with gap-filling intermediates, such as PCNA, RFC, and polymerases. Proximity labeling or immunoprecipitation of tagged repair factors can reveal dynamic interactions during the gap-filling step. Phosphoproteomics can uncover signaling events that regulate gap filling.
Imaging and live-cell analysis
Fluorescence microscopy of GFP-tagged PCNA or polymerases at sites of UV damage (localized UV irradiation) allows real-time visualization of gap-filling factor recruitment. FRAP (fluorescence recovery after photobleaching) can measure the turnover of gap-filling proteins at repair sites. These methods are powerful for studying the kinetics of GO:0006297 in living cells.
How CRISPR Can Be Used to Study GO:0006297 nucleotide-excision repair, DNA gap filling
Knockout
CRISPR knockout of core gap-filling genes such as POLD1, LIG1, or PCNA is typically lethal in dividing cells, but conditional or inducible knockout systems allow study of their roles in NER. Knockout of XPG or Cdt2 can be used to dissect the incision-to-gap-filling transition. EDITGENE provides validated knockout cell lines for these targets.
Point Mutation
Point mutations in polymerase active sites or in the XPG degron can be introduced by CRISPR to separate catalytic activity from regulatory functions. For example, a point mutation in the Cdt2-binding degron of XPG stabilizes the protein and impairs gap filling. Such models are valuable for studying disease-associated variants.
Knock-in
Knock-in of fluorescent tags (e.g., GFP, mCherry) at endogenous loci of PCNA, POLD1, or LIG1 enables live-cell imaging of gap-filling dynamics. Knock-in of disease-relevant mutations, such as POLE exonuclease domain mutations, creates isogenic models for cancer research. EDITGENE offers precision knock-in services.
Overexpression
Overexpression of gap-filling enzymes such as POLK or POLB can be achieved by lentiviral transduction or CRISPR activation (CRISPRa). Overexpression models help determine whether increased gap-filling capacity enhances resistance to DNA-damaging agents. EDITGENE provides stable overexpression cell lines for these studies.
How EDITGENE Supports nucleotide-excision repair, DNA gap filling Research
Researchers studying nucleotide-excision repair, DNA gap filling-related genes often need to determine whether a candidate gene is causally involved in the repair process, how mutations affect protein function, and whether the gene can be targeted therapeutically. This requires robust, isogenic cell models that recapitulate human genetics in a controlled setting.
Contact EDITGENE today to design your custom CRISPR model for nucleotide-excision repair, DNA gap filling research.
Frequently Asked Questions About nucleotide-excision repair, DNA gap filling
What is nucleotide-excision repair, DNA gap filling?
It is the step of nucleotide excision repair in which DNA polymerase fills the single-stranded gap left after the damaged DNA segment is removed, and DNA ligase seals the nick.
What genes are involved in nucleotide-excision repair, DNA gap filling?
Key genes include POLD1, POLE, POLK, PCNA, RFC, LIG1, LIG3, XPG, and Cdt2, among others.
What is the GO ID for nucleotide-excision repair, DNA gap filling?
The Gene Ontology ID is GO:0006297.
How does gap filling differ from base excision repair gap filling?
Both processes use DNA polymerases and ligases, but NER gap filling occurs after dual incision of a bulky lesion, whereas BER gap filling follows removal of a single damaged base.
Which DNA polymerase performs gap filling in nucleotide excision repair?
DNA polymerase delta and epsilon are the main gap-filling polymerases in replicating cells, with DNA polymerase kappa acting in some contexts.
What happens if gap filling fails during nucleotide excision repair?
Persistent single-strand breaks can be converted to double-strand breaks, leading to genomic instability, senescence, and cell death.
Is nucleotide-excision repair, DNA gap filling linked to cancer?
Yes, defects in NER gap filling are associated with increased cancer risk, including skin cancers and colorectal cancers with POLE mutations.
How is XPG involved in gap filling?
XPG performs the 3' incision, and its subsequent ubiquitination by Cdt2 promotes the transition to gap-filling DNA synthesis.
What experimental models are used to study nucleotide-excision repair, DNA gap filling?
Common models include knockout cell lines, point-mutation knock-ins, fluorescently tagged knock-ins, and overexpression lines, combined with DNA repair assays and sequencing.
Can CRISPR be used to study nucleotide-excision repair, DNA gap filling?
Yes, CRISPR knockout, point mutation, knock-in, and overexpression models are widely used to dissect the function of gap-filling genes.
Conclusion
GO:0006297, nucleotide-excision repair, DNA gap filling, is a critical biological process that ensures genome stability by restoring DNA after the removal of bulky lesions. It involves a coordinated interplay of DNA polymerases, PCNA, RFC, and DNA ligases, and is regulated by ubiquitin-mediated degradation of XPG. Defects in this process are linked to cancer, premature aging, and senescence-associated DNA damage. Researchers can leverage CRISPR-based knockout, point-mutation, knock-in, and overexpression models to study gap filling in detail. EDITGENE provides comprehensive services to support these studies, from custom cell line generation to CRISPR library screening and bioinformatics analysis.
References
- 1. Kemp MG. 2019. Damage removal and gap filling in nucleotide excision repair.. Enzymes 45:59-97 PMID: 31627883
- 2. Sancar A. 1996. DNA excision repair.. Annu Rev Biochem 65:43-81 PMID: 8811174
- 3. Beard WA et al.. 2019. DNA polymerase beta and other gap-filling enzymes in mammalian base excision repair.. Enzymes 45:1-26 PMID: 31627875
- 4. Wang D et al.. 2022. Active DNA demethylation promotes cell fate specification and the DNA damage response.. Science 378(6623):983-989 PMID: 36454826
- 5. Suzuki T et al.. 2025. Faulty Gap Filling in Nucleotide Excision Repair Leads to Double-Strand Break Formation in Senescent Cells.. J Invest Dermatol 145(1):32-41.e11 PMID: 38871024
- 6. Leibeling D et al.. 2006. Nucleotide excision repair and cancer.. J Mol Histol 37(5-7):225-38 PMID: 16855787
- 7. Han C et al.. 2015. Cdt2-mediated XPG degradation promotes gap-filling DNA synthesis in nucleotide excision repair.. Cell Cycle 14(7):1103-15 PMID: 25483071
- 8. Çağlayan M. 2020. Pol β gap filling, DNA ligation and substrate-product channeling during base excision repair opposite oxidized 5-methylcytosine modifications.. DNA Repair (Amst) 95:102945 PMID: 32853828
