--- title: "Retrovirus — AP Biology Definition & Exam Guide" description: "A retrovirus is an RNA virus that uses reverse transcriptase to make DNA, flipping the normal DNA-to-RNA flow and challenging the central dogma on the AP Bio exam." canonical: "https://fiveable.me/ap-bio/key-terms/retrovirus" type: "key-term" subject: "AP Biology" unit: "Unit 6" --- # Retrovirus — AP Biology Definition & Exam Guide ## Definition A retrovirus is a virus with an RNA genome that uses the enzyme reverse transcriptase to convert its RNA into DNA, which then integrates into the host cell's genome and gets expressed using normal transcription and translation machinery. ## What It Is A retrovirus is a virus whose [genetic material](/ap-bio/key-terms/genetic-material "fv-autolink") is RNA, not DNA. The catch is that it can't just hand its RNA straight to a ribosome and call it a day. First it uses an [enzyme](/ap-bio/unit-3/enzyme-catalysis/study-guide/Jg1jljQ8ZHUvcaKprPGy "fv-autolink") called **reverse transcriptase** to copy that RNA into DNA. That DNA then slips into the host cell's own genome, where it sits like a stowaway gene. Once integrated, the viral DNA gets read by the host cell exactly like any of its own genes. It's transcribed into mRNA, and that mRNA is translated into viral proteins on ribosomes (EK 6.4.A.1). The most famous example is HIV, which infects human T cells. So the gene expression you study in [Unit 6](/ap-bio/unit-6 "fv-autolink") (transcription, then translation through initiation, elongation, and termination) is the same machinery a retrovirus hijacks once its genome is in place. ## Why It Matters Retroviruses live in Unit 6: Gene Expression and Regulation, anchored to Topic 6.4 Translation. They connect to learning objective [AP Bio](/ap-bio "fv-autolink") 6.4.A, which explains how [genotype](/ap-bio/key-terms/genotype "fv-autolink") determines phenotype, because a retrovirus literally inserts a new genotype into the host and lets the host's translation machinery build the resulting proteins (EK 6.4.A.1, EK 6.4.A.3). The bigger conceptual payoff is the central dogma. The classic version says information flows DNA to RNA to protein. Retroviruses run one arrow backward (RNA to DNA), so they're the go-to example for showing that the original dogma was incomplete. That makes the term a clean test of whether you actually understand the direction of information flow, not just the steps. ## Connections ### [Reverse Transcriptase (Unit 6)](/ap-bio/key-terms/reverse-transcriptase) This is the enzyme that makes a retrovirus a retrovirus. It reads the RNA [genome](/ap-bio/key-terms/genome "fv-autolink") and builds a DNA copy, which is the exact step that runs against the normal DNA-to-RNA direction. ### Translation (Unit 6) After the viral DNA integrates and gets transcribed, the resulting [mRNA](/ap-bio/key-terms/mrna "fv-autolink") is translated on host ribosomes just like any cellular mRNA, using start codons, elongation, and stop codons. The virus borrows your translation machinery to build its own proteins. ### Genetic Code and Codons (Unit 6) Viral mRNA is read in the same three-nucleotide codons as host mRNA, starting at a [start codon](/ap-bio/key-terms/start-codon "fv-autolink") and ending at a stop codon. A retrovirus doesn't bring its own genetic code; it uses the universal one your cells already speak. ### [Eukaryotic Cells (Unit 6)](/ap-bio/key-terms/eukaryotic-cells) HIV targets human (eukaryotic) T cells, so the integrated viral genes are transcribed in the nucleus and translated in the cytoplasm or on the rough ER. Knowing eukaryotic gene expression lets you trace where each viral step happens. ## On the AP Exam Retroviruses show up in MCQs that ask you to trace the flow of genetic information in HIV-infected cells. A common stem gives you a researcher studying HIV in human T cells and asks which sequence correctly describes information flow. The right answer always starts with RNA being reverse transcribed to DNA, then transcribed to mRNA, then translated to protein. Another classic stem asks why retroviruses 'challenge the central dogma,' and the answer is that they run RNA to DNA, a direction the original dogma didn't include. You may also see a question about what machinery makes new viral proteins after integration; the answer is the host cell's ribosomes and translation machinery. A fluorescent-marker experiment can test the same idea, where protein appears only after a delay because reverse transcription, integration, transcription, and translation all have to happen first. ## retrovirus vs reverse transcriptase A retrovirus is the whole virus (RNA genome plus its protein machinery). Reverse transcriptase is just one enzyme that virus carries. The retrovirus is the package; reverse transcriptase is the tool inside it that copies RNA into DNA. ## Key Takeaways - A retrovirus has an RNA genome and uses reverse transcriptase to convert that RNA into DNA before anything else happens. - After conversion, the viral DNA integrates into the host genome and is expressed using the host's own transcription and translation machinery. - Retroviruses are the textbook example that challenges the original central dogma because they run information backward from RNA to DNA. - HIV is the most common retrovirus on the AP exam, and it infects human T cells. - The viral proteins are built on host ribosomes using the same genetic code, start codons, and stop codons as host genes (EK 6.4.A.1). ## FAQs ### What is a retrovirus in AP Biology? A retrovirus is a virus with an RNA genome that uses reverse transcriptase to make a DNA copy of itself. That DNA integrates into the host genome and is then transcribed and translated by the host cell to make viral proteins. ### Why does a retrovirus challenge the central dogma? The original central dogma says information flows DNA to RNA to protein. A retrovirus runs the first arrow in reverse, copying RNA into DNA with reverse transcriptase, which the original model didn't account for. ### Is a retrovirus the same thing as reverse transcriptase? No. A retrovirus is the entire virus, including its RNA genome and proteins. Reverse transcriptase is just the enzyme the virus uses to turn its RNA into DNA. The virus carries the enzyme; the enzyme is not the virus. ### What machinery makes new retroviral proteins after the virus integrates? The host cell's own machinery. Once the viral DNA is integrated, it's transcribed into mRNA and translated on the host ribosomes, exactly like the cell's normal genes (EK 6.4.A.1). ### Why is there a delay before infected cells show viral proteins? Because several steps have to happen first: the RNA gets reverse transcribed to DNA, the DNA integrates into the genome, then it's transcribed and finally translated. In a fluorescent-marker experiment, the protein only appears after all those steps finish, which is why it takes hours. ## Related Study Guides - [6.4 Translation](/ap-bio/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-bio/key-terms/retrovirus#resource","name":"Retrovirus — AP Biology Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/retrovirus","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/retrovirus#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:40.753Z","isPartOf":{"@type":"Collection","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-bio/key-terms/retrovirus#term","name":"retrovirus","description":"A retrovirus is a virus with an RNA genome that uses the enzyme reverse transcriptase to convert its RNA into DNA, which then integrates into the host cell's genome and gets expressed using normal transcription and translation machinery.","url":"https://fiveable.me/ap-bio/key-terms/retrovirus","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is a retrovirus in AP Biology?","acceptedAnswer":{"@type":"Answer","text":"A retrovirus is a virus with an RNA genome that uses reverse transcriptase to make a DNA copy of itself. 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