--- title: "Base Excision Repair | General Biology I" description: "Base excision repair fixes small DNA base damage by removing the damaged base, replacing the gap, and sealing the strand in General Biology I." canonical: "https://fiveable.me/college-bio/key-terms/base-excision-repair" type: "key-term" subject: "General Biology I" unit: "Unit 14" --- # Base Excision Repair | General Biology I ## Definition Base excision repair is a DNA repair pathway that fixes small, non-helix-distorting base damage in General Biology I. It removes the damaged base, fills the gap, and seals the DNA backbone. ## What It Is Base excision repair is the cell's way of fixing small, damaged DNA bases in General Biology I. It targets lesions that do not badly twist the double helix, such as bases altered by oxidation, deamination, or other normal chemical wear. The pathway starts when a DNA glycosylase finds a damaged base and cuts the bond between that base and the sugar in the DNA backbone. That leaves behind an abasic site, also called an AP site, which is a spot in the DNA where the base is missing. Next, an AP endonuclease cuts the sugar-phosphate backbone near that empty spot. From there, DNA polymerase adds the correct nucleotide, using the opposite strand as the template. DNA ligase then seals the final nick so the strand is continuous again. A helpful way to think about it is that base excision repair replaces one bad letter without rewriting the whole sentence. That is different from repair systems that remove a larger stretch of DNA. BER is built for the kinds of tiny chemical changes that happen all the time inside cells, especially from normal metabolism and reactive oxygen species. This pathway can be short-patch or long-patch repair. In short-patch BER, one nucleotide is replaced. In long-patch BER, a few nucleotides are removed and resynthesized before ligation. Either way, the goal is the same, which is to restore the original DNA sequence before the damage turns into a mutation during replication. ## Why It Matters Base excision repair shows up any time General Biology I connects DNA damage to mutation, genome stability, or cancer risk. If damaged bases are left in place, DNA polymerase may copy the wrong information during replication, and that error can become permanent in daughter cells. It also gives you a clean example of how cells use specialized enzymes in sequence. One enzyme recognizes the damage, another cuts the backbone, another fills in the correct nucleotide, and ligase finishes the job. That step-by-step logic is a big theme in cell and molecular biology, because many pathways work as coordinated enzyme chains rather than single reactions. This term also helps you compare repair pathways. When the damage is a small chemical change to one base, BER is the right fix. When the lesion distorts the helix more broadly, another pathway such as nucleotide excision repair is a better match. That distinction comes up often in quizzes and short-answer questions. In a broader genetics context, BER explains why cells are not passively exposed to mutation. They constantly monitor and correct DNA, which is one reason life can preserve genetic information even while cells face oxidative stress, heat, and chemical byproducts every day. ## Connections ### DNA glycosylase DNA glycosylases are the enzymes that start base excision repair. They scan DNA for a damaged base and remove that base first, which creates the abasic site that the rest of the pathway works on. If you are tracing the repair sequence, this is the recognition step that decides where the repair begins. ### [Apurinic/apyrimidinic site](/college-bio/key-terms/apurinicapyrimidinic-site) An apurinic/apyrimidinic site, or AP site, is the empty spot left after the damaged base is removed. It is not a full repair yet, just an intermediate that still needs backbone cutting, nucleotide replacement, and ligation. In diagrams, this is usually the point where the base is gone but the sugar-phosphate chain is still partly intact. ### [AP endonuclease](/college-bio/key-terms/ap-endonuclease) AP endonuclease works after the AP site forms. It cuts the DNA backbone near the missing base so the repair machinery can remove the damaged section and replace it. This enzyme links recognition to replacement, which makes it one of the most important middle steps in the pathway. ### Nucleotide excision repair Nucleotide excision repair is a different DNA repair pathway, and it usually handles larger, helix-distorting lesions. Base excision repair is for small base changes that do not warp the double helix much. If a question asks which pathway fixes tiny chemical damage versus bulky distortion, this is the comparison to make. ## On the AP Exam A quiz or exam question might give you a DNA damage scenario and ask which repair pathway fits best. If the lesion is a small base change from oxidation or deamination, you should identify base excision repair and then trace the order of enzymes: glycosylase, AP endonuclease, DNA polymerase, and ligase. You may also see it in a diagram and need to label the AP site or explain why a mutation would increase if the pathway fails. In short-answer questions, the safest move is to connect the repair step to genome stability: the cell removes one damaged base before replication turns that damage into a permanent change in the DNA sequence. ## Base excision repair vs Nucleotide excision repair These two pathways both fix DNA damage, but they handle different problems. Base excision repair fixes small, non-helix-distorting base lesions, while nucleotide excision repair removes bulkier damage that bends the DNA helix, like UV-related lesions. If the prompt emphasizes a single altered base, think BER. If it emphasizes a bulky distortion, think NER. ## Key Takeaways - Base excision repair fixes small chemical damage to individual DNA bases before the damage becomes a mutation. - The pathway begins when a DNA glycosylase removes the damaged base, creating an AP site. - AP endonuclease cuts the DNA backbone, DNA polymerase fills in the missing nucleotide, and DNA ligase seals the strand. - BER is especially useful for damage caused by oxidation, deamination, and other normal cellular processes. - If the damaged base is not repaired before replication, the change can be copied into daughter cells and raise mutation risk. ## FAQs ### What is base excision repair in General Biology I? Base excision repair is a DNA repair pathway that fixes small, non-helix-distorting base damage. The cell removes the damaged base, replaces the missing nucleotide, and seals the DNA backbone so the sequence is restored. ### What enzymes are involved in base excision repair? The main enzymes are DNA glycosylase, AP endonuclease, DNA polymerase, and DNA ligase. Glycosylase starts the process by removing the damaged base, AP endonuclease cuts the backbone, polymerase fills the gap, and ligase closes the strand. ### How is base excision repair different from nucleotide excision repair? Base excision repair fixes smaller base lesions that do not greatly distort the helix. Nucleotide excision repair removes a short stretch of DNA around larger, bulky lesions that bend or distort the double helix. That difference is a common way biology questions test pathway identification. ### Why does base excision repair matter for mutations? If a damaged base is left unrepaired, DNA replication can copy the wrong information into the new strand. That turns a temporary chemical lesion into a permanent mutation, which can affect gene function and raise cancer risk. ## Related Study Guides - [14.6 DNA Repair](/college-bio/unit-14/6-dna-repair/study-guide/8aVJvLnFf0yTIn3Z) ## 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/base-excision-repair#resource","name":"Base Excision Repair | General Biology I","url":"https://fiveable.me/college-bio/key-terms/base-excision-repair","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/base-excision-repair#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:07.934Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/base-excision-repair#term","name":"Base excision repair","description":"Base excision repair is a DNA repair pathway that fixes small, non-helix-distorting base damage in General Biology I. It removes the damaged base, fills the gap, and seals the DNA backbone.","url":"https://fiveable.me/college-bio/key-terms/base-excision-repair","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is base excision repair in General Biology I?","acceptedAnswer":{"@type":"Answer","text":"Base excision repair is a DNA repair pathway that fixes small, non-helix-distorting base damage. The cell removes the damaged base, replaces the missing nucleotide, and seals the DNA backbone so the sequence is restored."}},{"@type":"Question","name":"What enzymes are involved in base excision repair?","acceptedAnswer":{"@type":"Answer","text":"The main enzymes are DNA glycosylase, AP endonuclease, DNA polymerase, and DNA ligase. Glycosylase starts the process by removing the damaged base, AP endonuclease cuts the backbone, polymerase fills the gap, and ligase closes the strand."}},{"@type":"Question","name":"How is base excision repair different from nucleotide excision repair?","acceptedAnswer":{"@type":"Answer","text":"Base excision repair fixes smaller base lesions that do not greatly distort the helix. Nucleotide excision repair removes a short stretch of DNA around larger, bulky lesions that bend or distort the double helix. That difference is a common way biology questions test pathway identification."}},{"@type":"Question","name":"Why does base excision repair matter for mutations?","acceptedAnswer":{"@type":"Answer","text":"If a damaged base is left unrepaired, DNA replication can copy the wrong information into the new strand. That turns a temporary chemical lesion into a permanent mutation, which can affect gene function and raise cancer risk."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"General Biology I","item":"https://fiveable.me/college-bio"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/college-bio/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 14","item":"https://fiveable.me/college-bio/unit-14"},{"@type":"ListItem","position":4,"name":"Base excision repair"}]}]} ```