--- title: "DNA Polymerase III | Anatomy and Physiology I" description: "DNA polymerase III is the main enzyme that builds new DNA strands during replication in Anatomy and Physiology I, copying genetic material accurately." canonical: "https://fiveable.me/anatomy-physiology/key-terms/dna-polymerase-iii" type: "key-term" subject: "Anatomy and Physiology I" unit: "Unit 3" --- # DNA Polymerase III | Anatomy and Physiology I ## Definition DNA polymerase III is the enzyme that adds nucleotides to a new DNA strand during replication. In Anatomy and Physiology I, it is the main builder that copies the cell’s genetic information before division. ## What It Is DNA polymerase III is the main enzyme that extends a new DNA strand during DNA replication in Anatomy and Physiology I cell biology. It reads the template strand and adds complementary nucleotides, so the cell can make an accurate copy of its DNA before division. The enzyme works at the replication fork, where the double helix has already been opened by helicase. Once primase lays down a short RNA primer, DNA polymerase III can start adding DNA nucleotides to the primer’s free 3' end. That direction matters because DNA polymerases can only build in the 5' to 3' direction. This enzyme is fast and highly processive, which means it can stay attached to the template and keep adding many nucleotides without falling off. That matters because human DNA is long, and replication has to move efficiently if a cell is going to finish copying its genome before cell division. DNA polymerase III also proofreads. If it inserts the wrong nucleotide, it can detect the mismatch and remove the incorrect base before continuing. That proofreading step lowers mutation rates and helps preserve the genetic instructions needed for normal cell function. Because DNA is copied on two strands at once, DNA polymerase III behaves a little differently on the leading and lagging strands. On the leading strand, it moves smoothly toward the replication fork. On the lagging strand, it works in short bursts, helping build the segments that later get joined into a continuous strand. That coordination is why replication looks like a team effort, not a single-enzyme job. ## Why It Matters DNA polymerase III sits in the middle of the replication process, so it ties together several ideas from the nucleus unit: DNA structure, complementary base pairing, and cell division. If you know what this enzyme does, it becomes much easier to explain how one DNA molecule turns into two matching copies. It also gives you a clear way to think about errors in copying genetic material. When replication goes wrong, the result can be a mutation, and that can affect how a cell makes proteins or how well tissues function. In Anatomy and Physiology I, that connection helps you link molecular events to larger ideas like growth, repair, and normal body function. This term also helps you separate the steps of replication. Helicase opens the helix, primase starts the chain, DNA polymerase III extends it, and other enzymes finish the job. Once you can place DNA polymerase III in that sequence, replication questions become much easier to trace from start to finish. ## Connections ### DNA replication DNA polymerase III is one of the central enzymes in DNA replication, the process that copies a cell’s genetic material before division. If you are tracing the whole pathway, this is the step where the new strand actually grows. The enzyme only makes sense when you already know that replication begins after the DNA strands separate. ### Helicase Helicase comes before DNA polymerase III because it unwinds the double helix and opens the replication fork. Without that opening step, the polymerase cannot reach the template strand. A lot of class questions ask you to put these enzymes in order, so keep their jobs separate: helicase unzips, polymerase copies. ### Primase Primase works right before DNA polymerase III by laying down the RNA primer the polymerase needs to begin. DNA polymerase III cannot start a brand-new strand from nothing, so the primer gives it a free 3' end to extend. That makes primase the starter, and polymerase III the builder. ### [Okazaki Fragments](/anatomy-physiology/key-terms/okazaki-fragments) On the lagging strand, DNA polymerase III makes DNA in short sections called Okazaki fragments. This happens because the enzyme can only build in one direction, even though the fork is opening the other way. Those fragments are later joined into one continuous strand, so they show why replication needs multiple steps. ## On the AP Exam A quiz question might ask you to identify which enzyme extends a new DNA strand, or to put replication steps in order from helicase to primase to DNA polymerase III. In a diagram label or process question, you use this term to point to the enzyme working at the replication fork, not the enzyme that starts the primer or seals fragments. If the prompt asks why replication is accurate, mention proofreading and complementary base pairing. If it asks about the lagging strand, connect DNA polymerase III to Okazaki fragments and the need to build in short pieces. ## DNA polymerase III vs DNA polymerase I DNA polymerase III does the main strand-building during replication, while DNA polymerase I removes RNA primers and replaces them with DNA. They both work on the same overall process, which is why they get mixed up. A simple way to remember it is that polymerase III builds most of the new DNA, and polymerase I helps clean up and finish the strands. ## Key Takeaways - DNA polymerase III is the enzyme that adds nucleotides to a growing DNA strand during replication. - It works after helicase opens the helix and primase lays down an RNA primer. - The enzyme builds only in the 5' to 3' direction and proofreads for mistakes as it goes. - On the leading strand it moves continuously, but on the lagging strand it helps make Okazaki fragments. - In Anatomy and Physiology I, this term usually appears when you trace how cells copy DNA before division. ## FAQs ### What is DNA polymerase III in Anatomy and Physiology I? DNA polymerase III is the main enzyme that copies DNA during replication by adding complementary nucleotides to a new strand. In A&P I, you usually meet it in the nucleus and DNA replication unit, where it helps explain how cells prepare for division. ### How is DNA polymerase III different from DNA polymerase I? DNA polymerase III does most of the strand extension during replication, while DNA polymerase I mainly removes RNA primers and replaces them with DNA. If a question asks which enzyme builds the bulk of the new strand, choose polymerase III. ### Why can DNA polymerase III only build in one direction? DNA polymerase III can add nucleotides only to the 3' end of a growing strand, so new DNA is synthesized in the 5' to 3' direction. That is why the leading strand is continuous and the lagging strand has to be made in fragments. ### What does DNA polymerase III do on the lagging strand? On the lagging strand, DNA polymerase III makes short stretches of DNA called Okazaki fragments. The strand is still being copied accurately, but it has to happen in pieces because the enzyme cannot synthesize in the opposite direction. ## Related Study Guides - [3.3 The Nucleus and DNA Replication ](/anatomy-physiology/unit-3/3-nucleus-dna-replication/study-guide/Z6aQCDnl9bluwp72) ## 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/anatomy-physiology/key-terms/dna-polymerase-iii#resource","name":"DNA Polymerase III | Anatomy and Physiology I","url":"https://fiveable.me/anatomy-physiology/key-terms/dna-polymerase-iii","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/anatomy-physiology/key-terms/dna-polymerase-iii#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:20:38.182Z","isPartOf":{"@type":"Collection","name":"Anatomy and Physiology I Key Terms","url":"https://fiveable.me/anatomy-physiology/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/anatomy-physiology/key-terms/dna-polymerase-iii#term","name":"DNA polymerase III","description":"DNA polymerase III is the enzyme that adds nucleotides to a new DNA strand during replication. 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