--- title: "Mismatch Repair | General Biology I" description: "Mismatch repair fixes DNA replication mismatches by spotting mispaired bases and replacing the wrong stretch, helping preserve genome stability in General Biology I." canonical: "https://fiveable.me/college-bio/key-terms/mismatch-repair" type: "key-term" subject: "General Biology I" unit: "Unit 14" --- # Mismatch Repair | General Biology I ## Definition Mismatch repair is a DNA repair system that finds and fixes base-pair mismatches left behind after replication. In General Biology I, it shows how cells proofread newly made DNA and prevent mutations. ## What It Is Mismatch repair is a DNA repair pathway in General Biology I that fixes base-pairing mistakes left behind after DNA replication. When DNA polymerase inserts the wrong nucleotide, the double helix can still finish copying, but the new strand may contain a mismatch that would become a permanent mutation if the cell left it alone. The system does not usually look for a single wrong base in isolation. Instead, it detects a small distortion in the DNA double helix caused by an incorrect pairing, like G paired with T instead of G paired with C. That local shape change is the signal that tells repair proteins, "something is off here." In bacteria, proteins such as MutS recognize the mismatch, and MutL helps recruit the rest of the repair machinery. The cell then targets the newly made strand, removes the section containing the error, and fills the gap with the correct nucleotides. DNA ligase seals the final break so the strand is continuous again. A big idea in this topic is that mismatch repair works after replication, not before. DNA polymerase does have proofreading ability, but it cannot catch every mistake. Mismatch repair acts as a second pass, which is why it is often described as a post-replication editing system. This step matters because the cell has to correct the newly synthesized strand without damaging the original template. In many organisms, the repair system uses clues tied to the new DNA, such as temporary strand marks or nicks, to decide which strand should be changed. That strand choice is what keeps the original genetic information intact instead of accidentally "repairing" the correct base away. ## Why It Matters Mismatch repair sits right at the point where DNA replication errors become either temporary mistakes or lasting mutations. In General Biology I, it connects two big topics: how DNA is copied and how cells maintain genetic stability after copying is done. Without this pathway, small replication errors would build up fast, especially in cells that divide often. Even a single mismatch can become fixed in the genome after the next round of replication, turning a copying mistake into a true mutation. That is why this term shows up whenever the course talks about mutation rates, heredity, and DNA repair. It also helps explain why some defects in repair proteins have serious consequences. If mismatch repair does not work correctly, the cell accumulates extra mutations over time, which can contribute to cancer. Lynch syndrome is the classic human example you may see linked to this pathway. This term also shows up as a comparison point with other repair systems. Once you can separate mismatch repair from proofreading, base excision repair, or nucleotide excision repair, it becomes easier to identify what kind of DNA problem a cell is fixing and when the repair happens. ## Connections ### [DNA polymerase](/college-bio/key-terms/dna-polymerase) DNA polymerase is the enzyme that builds the new DNA strand, so mismatch repair deals with some of the mistakes it leaves behind. Polymerase starts the copying process, but if it inserts the wrong base, mismatch repair is one of the systems that can correct the error after replication finishes. ### [3' to 5' exonuclease activity](/college-bio/key-terms/3-5-exonuclease-activity) This is the proofreading activity built into some DNA polymerases. It removes a wrong nucleotide immediately during replication, before the strand is fully finished. Mismatch repair is a separate backup system that catches errors proofreading missed. ### [DNA ligase](/college-bio/key-terms/dna-ligase) After mismatch repair removes the damaged stretch and DNA polymerase fills in the correct nucleotides, DNA ligase seals the sugar-phosphate backbone. Without ligase, the repaired strand would still have a break even if the bases are now correct. ### [Base excision repair](/college-bio/key-terms/base-excision-repair) Base excision repair fixes damaged or chemically altered individual bases, while mismatch repair fixes copying errors that create wrong base pairs. They can both restore DNA integrity, but they target different problems and use different recognition steps. ## On the AP Exam A quiz question might show a DNA sequence or a replication scenario and ask you to identify which repair system fixes a mismatched base pair. You should trace the order of events: replication error, mismatch detection, removal of the incorrect stretch, replacement with the right nucleotides, and sealing by ligase. You may also see a short case about a mutation-prone cell line or a cancer-related disorder and need to connect the problem to defective mismatch repair. If the prompt asks why a cell still needs this pathway when DNA polymerase proofreads, the answer is that proofreading misses some errors and mismatch repair acts as a second pass after replication. On diagrams, look for a local kink or an incorrect base pair in newly synthesized DNA rather than a damaged base from radiation or chemicals. That visual clue usually points you toward mismatch repair instead of another DNA repair pathway. ## mismatch repair vs 3' to 5' exonuclease activity These are related but not the same. 3' to 5' exonuclease activity is part of DNA polymerase proofreading during replication, while mismatch repair happens afterward as a separate repair pathway. If the question is about the polymerase immediately removing a wrong nucleotide, think proofreading. If it is about the cell scanning finished DNA for a mismatch and fixing it later, think mismatch repair. ## Key Takeaways - Mismatch repair fixes base-pairing mistakes that slip past DNA replication. - It works after replication, using proteins that recognize distortions in the DNA helix. - The pathway removes the incorrect section of the new strand and replaces it with the right bases. - DNA ligase finishes the job by sealing the repaired DNA backbone. - When mismatch repair fails, mutation rates rise and cancer risk can increase. ## FAQs ### What is mismatch repair in General Biology I? Mismatch repair is the DNA repair system that corrects wrongly paired bases after replication. It helps keep the genome accurate by fixing copying mistakes before they become permanent mutations. ### How is mismatch repair different from proofreading? Proofreading happens during DNA synthesis, when DNA polymerase removes the wrong nucleotide right away. Mismatch repair happens after replication and fixes errors proofreading missed. They work as a double-check system. ### What proteins are involved in mismatch repair? In bacteria, MutS detects the mismatch and MutL helps recruit other repair proteins. The exact proteins differ in eukaryotes, but the overall logic stays the same: find the mismatch, remove the bad section, and resynthesize the correct DNA. ### Why does mismatch repair matter for cancer? If mismatch repair proteins do not work, replication errors build up in the genome. That extra mutation load can disrupt genes that control cell division, which is why defective mismatch repair is linked to cancers like Lynch syndrome. ## Related Study Guides - [14.6 DNA Repair](/college-bio/unit-14/6-dna-repair/study-guide/8aVJvLnFf0yTIn3Z) - [14.3 Basics of DNA Replication](/college-bio/unit-14/3-basics-dna-replication/study-guide/zPiHGrcGnVsz574S) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/mismatch-repair#resource","name":"Mismatch Repair | General Biology I","url":"https://fiveable.me/college-bio/key-terms/mismatch-repair","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/mismatch-repair#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:10.328Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/mismatch-repair#term","name":"mismatch repair","description":"Mismatch repair is a DNA repair system that finds and fixes base-pair mismatches left behind after replication. 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