--- title: "Type I Topoisomerases | General Biology I" description: "Type I topoisomerases cut one DNA strand to relax supercoiling without ATP, keeping replication moving smoothly in General Biology I." canonical: "https://fiveable.me/college-bio/key-terms/type-topoisomerases" type: "key-term" subject: "General Biology I" unit: "Unit 14" --- # Type I Topoisomerases | General Biology I ## Definition Type I topoisomerases are enzymes in General Biology I that relieve DNA supercoiling by cutting one strand, letting the DNA unwind, then resealing it without ATP. ## What It Is Type I topoisomerases are DNA-relief enzymes that fix torsional stress during prokaryotic DNA replication. When helicase opens the double helix at the replication fork, the DNA ahead of the fork gets overtwisted. Type I topoisomerases cut one strand of the DNA, let the helix rotate or pass a segment through, and then seal the break again. That single-strand cut is the big difference from enzymes that make double-strand cuts. Because only one strand is broken at a time, the DNA is not fully separated or permanently damaged. The enzyme forms a short-lived intermediate, does the strand pass, and then restores the phosphodiester backbone so the chromosome stays intact. These enzymes work without ATP. The energy stored in the supercoiled DNA itself is enough to drive the relaxation step, which is why Type I topoisomerases are often described as energy-saving compared with other topological enzymes. In a fast-moving replication fork, that matters because unwinding DNA creates more twisting ahead of the fork almost as soon as helicase opens new bases. In General Biology I, the key idea is not just that they cut DNA, but that they manage DNA shape. Supercoiling is a physical problem, not a base-pairing problem. If the DNA gets too tightly twisted, the replication machinery stalls, polymerases cannot keep copying efficiently, and the cell risks replication stress or breaks. You will usually see Type I topoisomerases discussed right beside DNA helicase, DNA polymerase III, and the replication fork. Helicase opens the helix, topoisomerase removes the tension created by opening it, and the polymerase can then synthesize the new strand. In prokaryotes, that coordination keeps bidirectional replication moving from the origin toward termination without the chromosome tangling itself into a mess. There are two common Type I subtypes. Type IA enzymes mainly relax negative supercoils and are often linked to bacteria. Type IB enzymes can relax both negative and positive supercoils by letting the DNA swivel. For a biology course, the main thing to remember is the action, cut one strand, relieve supercoiling, reseal, keep replication moving. ## Why It Matters Type I topoisomerases show up anytime you are tracing what happens at a prokaryotic replication fork. They explain why DNA can keep unwinding even though unwinding creates extra strain ahead of the fork. Without this relief step, the chromosome would become too tightly wound for the replication machinery to keep moving. This term also gives you a clean way to connect structure and function. DNA is not just a string of bases, it is a physical molecule that twists, coils, and stores tension. Type I topoisomerases make that visible in a very direct way, since they solve a mechanical problem by briefly changing the DNA backbone and then restoring it. In biology questions, this often shows up as a cause and effect chain. Helicase opens DNA, supercoiling increases, topoisomerase relaxes the strain, and replication continues. If you can explain that sequence, you can usually answer questions about why enzymes need to work together instead of acting one at a time. It also helps you compare enzyme types. If a prompt asks why Type I topoisomerases are different from Type II topoisomerases, or why a replication process needs more than just helicase and polymerase, this term is part of the explanation. It is one of the best examples of how cells manage DNA topology, not just DNA sequence. ## Connections ### DNA Supercoiling Type I topoisomerases exist because replication creates supercoiling. When helicase opens the helix, the DNA ahead of the fork becomes overwound, and that tension has to be released so copying can continue. If you understand supercoiling, the enzyme suddenly makes sense as a repair to DNA shape, not a repair to DNA sequence. ### [DNA Helicase](/college-bio/key-terms/dna-helicase) Helicase and Type I topoisomerases work as partners at the replication fork. Helicase separates the strands, but that unwinding increases strain in front of the fork. Topoisomerase relieves the strain so helicase does not run into a physical roadblock while the chromosome is being copied. ### [DNA Polymerase III](/college-bio/key-terms/dna-polymerase-iii) DNA polymerase III is the main enzyme building new DNA strands in prokaryotes, but it can only work well if the template is accessible. Type I topoisomerases keep the DNA from becoming too tightly twisted, which helps polymerase III move along the template without stalling. ### [Replication Fork](/college-bio/key-terms/replication-fork) The replication fork is the site where DNA is actively opening and being copied. Type I topoisomerases act just ahead of that fork, where torsional stress builds up fastest. If you are labeling a replication diagram, this is the spot where the enzyme’s job makes the most sense. ## On the AP Exam A quiz question may show a replication diagram and ask which enzyme prevents DNA ahead of the fork from becoming too tightly wound. You would identify Type I topoisomerase if the prompt describes a single-strand cut, relaxed supercoils, and no ATP use. In a short-answer or lab-style question, you might trace the order of events at a prokaryotic origin of replication, explaining how helicase creates tension and topoisomerase removes it. If a question asks why replication would slow or stop when this enzyme is missing, the answer is that supercoiling would build up and block smooth unwinding. On image-based questions, look for the enzyme acting in front of the replication fork, not behind it where DNA ligase seals fragments. ## Type I topoisomerases vs Type II topoisomerases Type I topoisomerases cut one DNA strand at a time and usually relax supercoils without ATP. Type II topoisomerases cut both strands, use ATP, and are better known for more dramatic DNA untangling and decatenation. If a question mentions a single-strand break and no ATP, think Type I. ## Key Takeaways - Type I topoisomerases relieve DNA supercoiling by making a temporary single-strand break in DNA and then resealing it. - In prokaryotic DNA replication, they work ahead of the replication fork where unwinding creates torsional strain. - They do not use ATP, because the DNA's own supercoiled tension powers the relaxation step. - They are different from Type II topoisomerases, which cut both DNA strands and usually use ATP. - If DNA replication stalls because the helix is too tightly wound, Type I topoisomerases are one of the first enzymes to think about. ## FAQs ### What is Type I topoisomerase in General Biology I? Type I topoisomerase is an enzyme that relaxes supercoiled DNA by cutting one strand, letting the DNA unwind a little, and then resealing the break. In General Biology I, you usually see it during prokaryotic DNA replication, where it prevents twisting stress from building up ahead of the replication fork. ### How does Type I topoisomerase differ from Type II topoisomerase? Type I topoisomerase cuts one DNA strand and usually does not need ATP. Type II topoisomerase cuts both strands and uses ATP, which lets it do more extensive untangling. If a question mentions a single-strand break, it is pointing to Type I. ### Why does DNA need topoisomerase during replication? Helicase opens the double helix, but that opening creates twisting strain in the DNA ahead of the replication fork. Topoisomerase removes that strain so the DNA can keep unwinding smoothly and the polymerase can keep copying the template. ### Where does Type I topoisomerase act in prokaryotic DNA replication? It acts ahead of the replication fork, where the DNA becomes overwound as the strands separate. This location matters because the enzyme is relieving mechanical stress created by helicase, not fixing the new DNA after it is made. ## Related Study Guides - [14.4 DNA Replication in Prokaryotes](/college-bio/unit-14/4-dna-replication-prokaryotes/study-guide/wkDB42LZyArVNAej) ## 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/type-topoisomerases#resource","name":"Type I Topoisomerases | General Biology I","url":"https://fiveable.me/college-bio/key-terms/type-topoisomerases","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/type-topoisomerases#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:11.425Z","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/type-topoisomerases#term","name":"Type I topoisomerases","description":"Type I topoisomerases are enzymes in General Biology I that relieve DNA supercoiling by cutting one strand, letting the DNA unwind, then resealing it without ATP.","url":"https://fiveable.me/college-bio/key-terms/type-topoisomerases","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 Type I topoisomerase in General Biology I?","acceptedAnswer":{"@type":"Answer","text":"Type I topoisomerase is an enzyme that relaxes supercoiled DNA by cutting one strand, letting the DNA unwind a little, and then resealing the break. 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