What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Double Strand Break Repair: Mechanisms and Pathways

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Double Strand Break Repair

A double strand break (DSB) is a form of DNA damage in which the sugar-phosphate backbones of both complementary strands are severed at sites that are physically close enough to cause the DNA molecule to separate into two fragments. This is distinct from a single strand break, where only one backbone is interrupted and the complementary strand remains intact as a template for repair. DSBs are widely regarded as the most cytotoxic form of DNA damage because they disrupt the physical continuity of the chromosome, threaten the integrity of genetic information, and, if misrepaired, can generate chromosomal rearrangements, deletions, or translocations that drive oncogenesis.

The severity of a DSB is compounded by the fact that, unlike other lesions, there is no intact complementary strand to serve as a template for accurate restoration of the original sequence. The cell therefore faces a fundamental problem: how to rejoin two broken DNA ends without losing genetic information. To solve this, eukaryotic cells have evolved two principal and mechanistically distinct pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ directly ligates the two broken ends together with minimal processing, making it fast and available throughout most of the cell cycle, but inherently error-prone. HR, by contrast, uses a homologous duplex—typically the sister chromatid—as a template to copy the missing sequence, achieving high-fidelity repair but requiring the presence of a homologous template and therefore being restricted to the S and G2 phases of the cell cycle.

The choice between these pathways is not random; it is governed by cell cycle position, the nature of the DNA ends, and a network of regulatory proteins that control the initiation of DNA end resection. Understanding the molecular choreography of DSB repair is essential not only for a fundamental appreciation of genome stability but also for interpreting the mechanisms of cancer predisposition syndromes and the rationale behind modern cancer therapies that exploit DSB repair defects. This article provides a comprehensive overview of DSB repair, from the sources of damage to the clinical consequences of repair failure.

Types of Double Strand Breaks and Their Causes

DSBs arise from a variety of endogenous and exogenous sources, and the nature of the break influences which repair pathway is engaged.

Endogenous Sources

Replication errors: The most frequent endogenous source of DSBs is the encounter between a replication fork and a lesion on the template strand. When a replicative DNA polymerase stalls at a damaged base, the fork can collapse, generating a one-ended DSB. These replication-associated DSBs are particularly problematic in regions of the genome that are difficult to replicate, such as common fragile sites, telomeres, and highly transcribed genes. The enzyme topoisomerase can also become trapped on DNA when it attempts to relieve torsional stress, creating a protein-linked DSB that requires specialized processing.

Reactive oxygen species (ROS): Normal cellular metabolism generates ROS, including superoxide, hydrogen peroxide, and hydroxyl radicals. These species can abstract hydrogen atoms from the deoxyribose sugar backbone of DNA, leading to sugar fragmentation and strand scission. When two such lesions occur in close proximity on opposite strands, a DSB results. The hydroxyl radical is particularly damaging and can produce clustered lesions—multiple damaged sites within one or two helical turns—that are difficult to repair.

Programmed DSBs: In a limited set of biological contexts, DSBs are generated deliberately. During V(D)J recombination in developing lymphocytes, the RAG1/RAG2 endonuclease introduces DSBs at recombination signal sequences to generate the diversity of immunoglobulin and T-cell receptor genes. Similarly, during meiosis, the Spo11 protein creates programmed DSBs that initiate homologous recombination between homologous chromosomes, a process essential for proper chromosome segregation and genetic diversity.

Exogenous Sources

Ionizing radiation (IR): X-rays, gamma rays, and particle radiation deposit energy in tissue, causing both direct ionization of DNA and indirect damage through the radiolysis of water, which generates ROS. A dose of 1 Gy of ionizing radiation induces approximately 20–40 DSBs per cell, alongside a much larger number of single strand breaks and base damages. The linear energy transfer (LET) of the radiation determines the complexity of the damage; high-LET radiation (e.g., alpha particles) produces more clustered, complex DSBs that are harder to repair.

Chemotherapeutic agents: Several classes of chemotherapy drugs induce DSBs. Topoisomerase II poisons such as etoposide and doxorubicin stabilize the covalent topoisomerase II-DNA cleavage complex, converting transient enzyme intermediates into persistent DSBs. Radiomimetic agents like bleomycin generate free radicals that produce DSBs directly. These agents are used clinically to exploit the fact that cancer cells are often deficient in DSB repair or are proliferating rapidly and therefore more sensitive to DNA damage.

Ultraviolet (UV) radiation: UV light primarily causes pyrimidine dimers and 6-4 photoproducts, which are repaired by nucleotide excision repair. However, when a replication fork encounters an unrepaired UV photoproduct, the resulting replication fork collapse can generate a DSB. Thus, UV is an indirect but significant source of DSBs in dividing cells.

Non-Homologous End Joining (NHEJ)

NHEJ is the dominant DSB repair pathway in mammalian cells, particularly in the G1 phase of the cell cycle when no sister chromatid is available for homologous recombination. It is a relatively simple process in concept—the two broken ends are brought together and ligated—but the molecular execution requires a coordinated series of protein associations and conformational changes. NHEJ is inherently error-prone because it often involves the loss or addition of a few nucleotides at the junction, but it is rapid and can operate on virtually any DSB, regardless of the sequence context.

NHEJ Steps

End recognition and binding by Ku: The first step is the binding of the Ku70/Ku80 heterodimer to the broken DNA ends. Ku is a ring-shaped protein that threads onto the DNA duplex, and it has an extraordinarily high affinity for DNA ends, with a dissociation constant in the picomolar range. Ku binding protects the ends from degradation and serves as a platform for recruiting downstream factors. The Ku heterodimer can bind to both ends of a DSB, and its presence is required for essentially all NHEJ events.

Recruitment and activation of DNA-PKcs: The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is recruited to the Ku-bound DNA ends. DNA-PKcs is a large serine/threonine kinase (approximately 470 kDa) that, together with Ku and DNA, forms the active DNA-PK holoenzyme. Binding to DNA ends activates the kinase activity of DNA-PKcs, leading to autophosphorylation of multiple sites on DNA-PKcs itself and phosphorylation of downstream targets. Autophosphorylation induces a conformational change that allows the two DNA ends to be brought into close proximity, a process called synapsis. The kinase activity of DNA-PKcs is essential for NHEJ, as inhibition of its catalytic activity blocks repair.

End processing: Many DSBs do not have ligatable ends. They may contain damaged nucleotides, hairpin structures, or overhangs that must be removed or modified before ligation can occur. The nuclease Artemis, which possesses both 5'→3' and 3'→5' exonuclease activities as well as an endonuclease activity, is recruited to the DNA-PK complex. Artemis is activated by phosphorylation by DNA-PKcs and can open hairpin structures and trim overhanging single-stranded DNA. Additional processing enzymes, including polynucleotide kinase phosphatase (PNKP) and DNA polymerases μ and λ, can add or remove phosphate groups and fill in gaps, respectively. The extent of processing is variable and determines the degree of sequence loss at the junction.

Ligation: The final step is the ligation of the two DNA ends by the XRCC4-DNA ligase IV complex. DNA ligase IV is specifically dedicated to NHEJ and cannot be replaced by other ligases. XRCC4 stabilizes ligase IV and stimulates its activity. The XRCC4-like factor XLF (also called Cernunnos) interacts with XRCC4 and is required for efficient ligation, particularly when the ends are not perfectly compatible. The ligation reaction requires ATP and magnesium ions, and it proceeds through a covalent DNA-adenylate intermediate before the phosphodiester bond is formed.

Removal of repair factors: After ligation is complete, the repair factors must be removed from the DNA. The Ku heterodimer is trapped on the DNA after ligation and must be actively removed, a process that involves the ubiquitin ligase RNF8 and the proteasome. Failure to remove Ku can interfere with subsequent DNA transactions such as transcription and replication.

NHEJ in Immune System Development

The physiological importance of NHEJ is nowhere more evident than in the development of the adaptive immune system. V(D)J recombination, the process that generates the enormous diversity of antigen receptor genes, relies on the introduction of programmed DSBs by the RAG1/RAG2 endonuclease and their subsequent repair by NHEJ. The RAG proteins introduce a DSB between the coding segment and the recombination signal sequence, generating a hairpin-sealed coding end and a blunt signal end. The coding ends are opened by Artemis (in a DNA-PKcs-dependent manner), processed, and joined by the NHEJ machinery, creating the junctional diversity that is essential for antigen receptor variability. Mutations in any of the core NHEJ factors—Ku, DNA-PKcs, Artemis, XRCC4, or ligase IV—cause severe combined immunodeficiency (SCID) in humans and mice, underscoring the non-redundant role of NHEJ in this process.

Homologous Recombination (HR)

Homologous recombination is the high-fidelity DSB repair pathway that uses a homologous DNA sequence as a template to restore the genetic information lost at the break. In somatic cells, the sister chromatid is the preferred template, which restricts HR to the S and G2 phases of the cell cycle when sister chromatids are available. HR is a complex, multi-step process that requires the coordinated action of numerous proteins, many of which are tumor suppressors.

HR Steps

End resection: The initiating and committing step of HR is the resection of the 5' ends at the DSB to generate long 3' single-stranded DNA (ssDNA) overhangs. Resection is a two-step process. First, the MRN complex (MRE11-RAD50-NBS1) together with CtIP (in mammals) initiates short-range resection, trimming back approximately 100–300 nucleotides. MRE11 possesses both endonuclease and 3'→5' exonuclease activities, and it is thought to make an endonucleolytic incision on the 5' strand, followed by exonucleolytic digestion back toward the break. Second, long-range resection is carried out by the exonuclease EXO1 (5'→3') and/or the helicase-nuclease BLM-DNA2 complex, which can resect thousands of nucleotides. The resulting 3' ssDNA overhangs are immediately coated by the ssDNA-binding protein RPA, which protects the ssDNA from degradation and prevents the formation of secondary structures.

RAD51 filament formation: The recruitment of RAD51 to the resected ssDNA is the central event in HR. RAD51 is a recombinase that polymerizes on ssDNA to form a helical nucleoprotein filament. This process requires the displacement of RPA by RAD51, which is energetically unfavorable and requires the action of mediator proteins. In mammals, the key mediators are BRCA2, which directly interacts with RAD51 and promotes its loading onto RPA-coated ssDNA, and the RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3), which stabilize the filament. BRCA2 is essential for HR; its loss results in severe HR deficiency and is associated with hereditary breast and ovarian cancer.

Strand invasion and D-loop formation: The RAD51-ssDNA filament searches the genome for a homologous duplex sequence. This search is a dynamic, ATP-dependent process in which the filament samples many DNA sequences before finding a match. Once homology is found, the filament invades the homologous duplex, displacing one strand and forming a displacement loop (D-loop). The invading 3' end is then used as a primer for DNA synthesis, extending the invading strand using the homologous duplex as a template. This DNA synthesis is carried out by a replicative polymerase, likely DNA polymerase δ or ε, in conjunction with PCNA and RFC.

Second-end capture and double Holliday junction formation: The extended invading strand can then anneal to the second resected end of the DSB, a process called second-end capture. This annealing generates a double Holliday junction (dHJ), a structure in which the two homologous duplexes are linked by two crossovers. The dHJ is then processed by DNA synthesis and ligation to seal all nicks.

Resolution and dissolution: The double Holliday junction must be resolved to separate the two recombined molecules. This can occur by two mechanisms. Resolution involves the endonucleolytic cleavage of the Holliday junctions by structure-specific nucleases, including MUS81-EME1, GEN1, and SLX1-SLX4. Depending on which strands are cleaved, resolution can produce either crossover or non-crossover products. Alternatively, dissolution is mediated by the BLM helicase in complex with TOPOIIIα and RMI1/RMI2. Dissolution involves the convergent branch migration of the two Holliday junctions, followed by the decatenation of the resulting hemicatenane by topoisomerase IIIα. Dissolution always produces non-crossover products, which is important for avoiding loss of heterozygosity in somatic cells.

HR in S/G2 Phase

The restriction of HR to the S and G2 phases is enforced at multiple levels. The most fundamental requirement is the presence of a sister chromatid, which is generated during DNA replication in S phase. However, the cell cycle regulation of HR is also controlled by cyclin-dependent kinases (CDKs). CDK activity is low in G1 and high in S/G2, and CDK-dependent phosphorylation of CtIP is required for the initiation of resection. In G1, low CDK activity prevents CtIP phosphorylation, blocking resection and thereby channeling repair toward NHEJ. Additionally, the expression of several HR genes, including RAD51 and BRCA2, is cell cycle-regulated, with peak expression in S/G2.

Choice Between NHEJ and HR

The decision to repair a DSB by NHEJ or HR is a critical regulatory point that determines the fidelity of repair and the risk of genomic instability. This choice is governed primarily by the initiation of DNA end resection, which commits the cell to HR and simultaneously inhibits NHEJ.

The Resection Switch

The key regulatory step is the competition between two proteins at the DSB: 53BP1 and BRCA1. 53BP1 is a large chromatin-binding protein that is recruited to DSBs in G1 phase. It promotes NHEJ by protecting the DNA ends from resection. 53BP1 achieves this by recruiting downstream effectors, including RIF1 and the shieldin complex, which block the access of resection nucleases to the DNA ends. In S/G2 phase, BRCA1 antagonizes 53BP1. BRCA1 is recruited to DSBs in a manner that depends on the cell cycle and the presence of sister chromatids, and it promotes resection by displacing 53BP1 and its effectors from the break site. The balance between 53BP1 and BRCA1 is therefore a major determinant of pathway choice.

Additional Regulatory Factors

The nature of the DNA ends also influences pathway choice. DSBs with chemically modified or damaged ends that cannot be directly ligated are more likely to undergo resection and be repaired by HR. The presence of a replication fork at the break site also favors HR, as the fork provides a natural template and the one-ended nature of the break cannot be repaired by NHEJ. Additionally, the local chromatin environment and the activity of chromatin remodelers can influence the accessibility of the break site to repair factors.

Cell Cycle Phase

The cell cycle phase is the dominant factor in pathway choice. In G1, when CDK activity is low and no sister chromatid is present, NHEJ is the only viable option. In S/G2, both pathways are available, and the choice is made based on the competition between 53BP1 and BRCA1. In practice, most DSBs in S/G2 are repaired by HR, particularly those that arise from replication fork collapse. However, NHEJ can still operate in S/G2, and the relative usage of the two pathways varies depending on the cell type and the nature of the damage.

Feature	NHEJ	HR
Template requirement	None	Homologous duplex (sister chromatid)
Cell cycle phase	All phases (dominant in G1)	S and G2
Fidelity	Error-prone (small insertions/deletions)	High-fidelity (error-free)
Key proteins	Ku70/Ku80, DNA-PKcs, Artemis, XRCC4, Ligase IV	MRN, CtIP, EXO1, BLM, BRCA1/2, RAD51
End resection	Minimal or none	Extensive (5'→3')
Speed	Fast (minutes)	Slow (hours)
Primary function	Immune system development, repair of IR-induced breaks	Repair of replication-associated breaks, meiosis

Methods to Study Double Strand Break Repair

Studying DSB repair requires methods to both detect the damage and quantify the repair activity. Several complementary techniques are commonly used in research and clinical diagnostics.

Immunofluorescence for γH2AX

One of the most sensitive and widely used methods to detect DSBs is immunofluorescence staining for phosphorylated histone H2AX (γH2AX). Within minutes of DSB formation, the ATM kinase phosphorylates H2AX at serine 139 in the chromatin flanking the break, generating γH2AX foci that can be visualized by fluorescence microscopy using a phospho-specific antibody. Each γH2AX focus corresponds to one DSB, allowing quantification of damage. The disappearance of γH2AX foci over time reflects the kinetics of repair. This method is highly sensitive and can detect single DSBs, but it requires careful controls and is semi-quantitative.

Comet Assay (Single-Cell Gel Electrophoresis)

The comet assay is a simple and rapid method to measure DNA damage, including DSBs, 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 out of the nucleus toward the anode, forming a "comet tail" whose length and intensity are proportional to the amount of DNA damage. The neutral comet assay is specifically used to detect DSBs, while the alkaline version detects both single and double strand breaks. The comet assay is inexpensive and requires minimal equipment, but it is less sensitive than γH2AX staining and cannot distinguish between different types of DSBs.

Reporter Assays (DR-GFP)

Reporter assays provide a quantitative measure of HR or NHEJ activity in living cells. The most commonly used HR reporter is DR-GFP, which contains two copies of the GFP gene: one full-length but inactive copy with an I-SceI endonuclease site, and one truncated copy. Expression of I-SceI introduces a DSB at the engineered site. If the break is repaired by HR using the truncated GFP copy as a template, the full-length GFP gene is restored, and the cell becomes green fluorescent. The percentage of GFP-positive cells, measured by flow cytometry, is directly proportional to HR activity. Similar reporters exist for NHEJ, in which the repair of an I-SceI-induced break restores a functional GFP gene only if the ends are rejoined with the correct reading frame. These assays are powerful because they provide a direct, quantitative readout of repair pathway usage.

CRISPR-Based Approaches

The CRISPR-Cas9 system has revolutionized DSB repair research by allowing the introduction of DSBs at virtually any genomic location with high specificity. Cas9 is an RNA-guided endonuclease that generates a blunt DSB at a site determined by a guide RNA. By delivering Cas9 and guide RNAs to cells, researchers can create defined DSBs and then monitor the repair outcomes by sequencing the junction regions. This approach has been used to map the determinants of NHEJ versus HR usage across the genome, to identify genes that influence repair pathway choice, and to generate precise genome edits for therapeutic purposes. High-throughput CRISPR screens, in which thousands of genes are knocked out in parallel and the effect on DSB repair is measured, have identified numerous novel regulators of NHEJ and HR.

Clinical Relevance and Diseases Linked to DSB Repair Defects

Defects in DSB repair are associated with a broad spectrum of human diseases, most notably cancer predisposition and inherited syndromes characterized by genomic instability, immunodeficiency, and neurodegeneration.

Cancer Predisposition: BRCA1 and BRCA2

The most well-known DSB repair genes in the context of cancer are BRCA1 and BRCA2. Germline mutations in either gene confer a high lifetime risk of breast and ovarian cancer, with BRCA1 mutation carriers having a 60–80% lifetime risk of breast cancer and a 40–60% risk of ovarian cancer. Both BRCA1 and BRCA2 are essential for HR, but they function at different steps. BRCA1 is involved in the early steps of resection and in the recruitment of downstream factors, while BRCA2 directly delivers RAD51 to the resected ssDNA. Loss of either protein results in HR deficiency, leading to the accumulation of unrepaired or misrepaired DSBs and genomic instability. The HR deficiency of BRCA-mutant tumors is the basis for the therapeutic use of PARP inhibitors, which are selectively toxic to HR-deficient cells due to the concept of synthetic lethality.

Ataxia Telangiectasia (A-T)

Ataxia telangiectasia is a rare autosomal recessive disorder caused by mutations in the ATM gene, which encodes a master kinase that coordinates the cellular response to DSBs. ATM is activated by DSBs and phosphorylates numerous substrates, including H2AX, 53BP1, BRCA1, and p53, to initiate cell cycle checkpoints, DNA repair, and apoptosis. Patients with A-T exhibit progressive cerebellar neurodegeneration, immunodeficiency, radiosensitivity, and a high incidence of lymphoid malignancies. The neurodegeneration is thought to result from the accumulation of unrepaired DSBs in post-mitotic neurons, which cannot undergo HR and are particularly dependent on NHEJ.

Nijmegen Breakage Syndrome (NBS)

Nijmegen breakage syndrome is caused by mutations in NBS1, a component of the MRN complex. The MRN complex is the primary sensor of DSBs and is required for ATM activation and for the initiation of resection. Patients with NBS exhibit microcephaly, immunodeficiency, radiosensitivity, and a strong predisposition to lymphoma. The clinical features overlap with A-T, reflecting the shared role of ATM and MRN in DSB signaling.

Other DSB Repair Disorders

Mutations in other DSB repair genes cause additional syndromes. Mutations in the MRE11 gene cause ataxia telangiectasia-like disorder (ATLD), which resembles A-T but is milder. Mutations in DNA ligase IV cause a syndrome characterized by microcephaly, immunodeficiency, and radiosensitivity. Mutations in Artemis cause a form of SCID with radiation sensitivity. The diversity of these syndromes underscores the essential role of DSB repair in development, immunity, and genome maintenance.

Common Pitfalls and Misconceptions in DSB Repair

Students frequently encounter several conceptual difficulties when learning about DSB repair. Being aware of these pitfalls can help avoid common errors on exams and in research.

Confusing NHEJ and HR: The most common error is to conflate the two pathways. Remember that NHEJ does not use a template and is error-prone, while HR uses a homologous template and is error-free. NHEJ is fast and works in G1; HR is slow and requires S/G2. A useful mnemonic: "NHEJ = No Homology, Error-prone, Joining; HR = Homology Required."

Misunderstanding resection: Resection is the 5'→3' degradation of DNA ends to generate 3' overhangs. It is the committing step for HR and is inhibited by 53BP1. Students often incorrectly think that resection generates 5' overhangs or that it is required for NHEJ. Resection is the key switch that determines pathway choice.

Overlooking cell cycle regulation: The cell cycle phase is the primary determinant of pathway choice. NHEJ can occur at any time, but HR is restricted to S/G2. Students sometimes forget that HR requires a sister chromatid and therefore cannot occur in G1. The CDK-dependent regulation of CtIP is a specific mechanism that enforces this restriction.

Thinking NHEJ is always mutagenic: While NHEJ is error-prone, it can be accurate when the ends are compatible and require no processing. The error rate of NHEJ is low for simple blunt-ended breaks but increases when ends require extensive processing. The pathway is "potentially mutagenic" rather than "always mutagenic."

Ignoring the role of RPA: RPA coats ssDNA during resection and must be displaced by RAD51 for HR to proceed. Students often forget that RPA is present on ssDNA before RAD51 loading and that BRCA2 is required for the exchange.

Confusing DSB repair with other repair pathways: DSB repair is distinct from Base Excision Repair, Mismatch Repair, and Single Strand Break Repair. These pathways deal with base damage, mismatches, and single strand nicks, respectively, and do not involve the rejoining of two broken DNA ends. The DNA Repair overview provides a useful framework for understanding how DSB repair fits into the broader context of genome maintenance.

Forgetting the role of helicases: The Helicase Break Hydrogen Bonds concept is relevant to HR, where helicases such as BLM unwind DNA to facilitate resection and branch migration. Helicases do not break phosphodiester bonds; they break hydrogen bonds between base pairs, which is a distinct activity from the nucleases that degrade DNA.

Summary and Key Takeaways

Double strand break repair is a critical cellular process that maintains genome stability and prevents cancer. The two major pathways, NHEJ and HR, are mechanistically distinct and are chosen based on cell cycle phase and the availability of a homologous template. NHEJ is fast, error-prone, and active throughout the cell cycle, while HR is slow, high-fidelity, and restricted to S/G2. The decision between the two pathways is governed by the initiation of DNA end resection, which is controlled by the antagonistic actions of 53BP1 and BRCA1. Defects in DSB repair cause severe human diseases, including cancer predisposition syndromes and immunodeficiency disorders. Understanding the mechanisms of DSB repair is essential for interpreting the molecular basis of these diseases and for developing targeted therapies.

Frequently Asked Questions

What is double strand break repair?

Double strand break repair is the collection of cellular processes that detect and repair DNA double strand breaks, where both strands of the DNA duplex are severed. The two principal pathways are non-homologous end joining (NHEJ), which directly ligates the broken ends, and homologous recombination (HR), which uses a homologous template to accurately restore the sequence.

What are the steps of double strand break repair?

The steps of NHEJ are: (1) Ku70/Ku80 binding to the DNA ends, (2) recruitment and activation of DNA-PKcs, (3) end processing by Artemis and other enzymes, (4) ligation by XRCC4-ligase IV, and (5) removal of repair factors. The steps of HR are: (1) 5' end resection to generate 3' ssDNA overhangs, (2) RPA coating and RAD51 filament formation, (3) strand invasion and D-loop formation, (4) DNA synthesis and second-end capture, and (5) resolution or dissolution of Holliday junctions.

What is the difference between NHEJ and HR?

NHEJ does not require a homologous template and directly ligates the broken ends, often with small insertions or deletions. It is fast and operates throughout the cell cycle. HR requires a homologous template (the sister chromatid) and is restricted to S/G2. It is slow but high-fidelity, restoring the original sequence without error.

Can you provide a diagram of double strand break repair?

A diagram would show a DSB with two broken ends. In NHEJ, the ends are bound by Ku, processed, and ligated directly. In HR, the ends are resected to generate 3' overhangs, which are coated by RPA and then RAD51. The RAD51 filament invades a homologous duplex, forming a D-loop, and DNA synthesis restores the sequence. The final products are either non-crossover (from dissolution) or crossover (from resolution).

What are examples of double strand break repair?

Examples include the repair of ionizing radiation-induced DSBs by NHEJ in G1 cells, the repair of replication fork collapse by HR in S phase, V(D)J recombination during immune system development, and meiotic recombination during gamete formation.

What is the process of double strand break repair?

The process begins with the detection of the break by sensor proteins (Ku in NHEJ, MRN in HR). This is followed by the recruitment of downstream factors, the decision to repair by NHEJ or HR, and the execution of the chosen pathway. The process is regulated by cell cycle checkpoints and the DNA damage response, which coordinate repair with cell cycle progression.

What are the types of double strand break repair?

The two main types are non-homologous end joining (NHEJ) and homologous recombination (HR). A variant of NHEJ called microhomology-mediated end joining (MMEJ) also exists, which uses short regions of homology to align the ends before ligation. HR can also proceed through alternative sub-pathways, including synthesis-dependent strand annealing (SDSA) and break-induced replication (BIR).

Key Takeaways

Double strand breaks are the most cytotoxic form of DNA damage, disrupting chromosome continuity and threatening genome stability.
NHEJ is the dominant repair pathway in G1, is fast and error-prone, and requires Ku, DNA-PKcs, Artemis, XRCC4, and ligase IV.
HR is the high-fidelity pathway in S/G2, requires a sister chromatid template, and depends on resection, RAD51 filament formation, and BRCA1/BRCA2.
The choice between NHEJ and HR is controlled by cell cycle phase and the competition between 53BP1 (promotes NHEJ) and BRCA1 (promotes HR) at the break site.
DNA end resection is the committing step for HR and is inhibited in G1 by 53BP1 and the shieldin complex.
Defects in DSB repair cause cancer predisposition (BRCA1/2), ataxia telangiectasia (ATM), and Nijmegen breakage syndrome (NBS1).
Experimental methods to study DSB repair include γH2AX immunofluorescence, the comet assay, DR-GFP reporter assays, and CRISPR-based approaches.
Understanding DSB repair is essential for interpreting cancer biology and for the rational design of therapies such as PARP inhibitors.