--- title: "Elongation — AP Bio Definition & Exam Guide" description: "Elongation is the middle stage of translation where amino acids get added one by one to a growing polypeptide. Learn how it fits CED topic 6.4 and the exam." canonical: "https://fiveable.me/ap-bio/key-terms/elongation" type: "key-term" subject: "AP Biology" unit: "Unit 6" --- # Elongation — AP Bio Definition & Exam Guide ## Definition In AP Bio, elongation is the second stage of translation, where the ribosome moves along the mRNA codon by codon and adds amino acids one at a time to the growing polypeptide chain via peptide bonds. ## What It Is Elongation is the middle act of [translation](/ap-bio/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh "fv-autolink"). Initiation gets the [ribosome](/ap-bio/key-terms/ribosome "fv-autolink") locked onto the mRNA at the start codon, then elongation does the actual building. The ribosome reads the mRNA one codon (three nucleotides) at a time, matching each codon to a tRNA carrying the correct amino acid. Each new amino acid gets joined to the chain by a peptide bond, and the ribosome ratchets forward to the next codon. Repeat, repeat, repeat until a stop codon shows up and termination ends the process. Per **EK 6.4.A.3**, translation runs through sequential steps: initiation, elongation, and termination. Elongation is the part that physically grows the **polypeptide**. It happens on [ribosomes](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9 "fv-autolink") in the cytoplasm of both prokaryotic and eukaryotic cells, and on the rough ER surface in eukaryotes (**EK 6.4.A.1**). One quick reframe: elongation is the assembly line, and each codon is a barcode telling the cell which amino acid to bolt on next. ## Why It Matters Elongation lives in **[Unit 6](/ap-bio/unit-6 "fv-autolink"): Gene Expression and Regulation**, specifically topic **6.4 Translation**. It supports **[AP Bio](/ap-bio "fv-autolink") 6.4.A**, which asks you to explain how an organism's phenotype comes from its genotype. The logic chain is the whole point of Unit 6: DNA holds the instructions, mRNA carries them out, and translation (with elongation doing the heavy lifting) turns that code into actual proteins that do jobs in the cell. If you can't explain how the message becomes a functional polypeptide, you can't connect genotype to phenotype, which is the big theme the exam keeps coming back to. ## Connections ### Polypeptide and the Genetic Code (Unit 6) Elongation is the moment the [genetic code](/ap-bio/key-terms/genetic-code "fv-autolink") actually gets read. Each three-nucleotide codon is matched to one amino acid, and elongation strings those amino acids into a polypeptide. The codon order in the mRNA directly sets the amino acid order in the protein. ### Prokaryotes vs. Eukaryotes in Translation (Unit 6) In [prokaryotes](/ap-bio/key-terms/prokaryotes "fv-autolink") translation happens while transcription is still going (EK 6.4.A.2), so elongation can start before the mRNA is even finished. In eukaryotes the mRNA gets processed and exported first, so elongation happens on cytoplasmic or rough-ER ribosomes after transcription is done. ### Reverse Transcriptase and Retroviruses (Unit 6) Retroviruses flip the usual flow by using [reverse transcriptase](/ap-bio/key-terms/reverse-transcriptase "fv-autolink") to make DNA from RNA. That DNA still has to be transcribed and translated normally, so elongation is the same process for viral proteins as for the host's own. ## On the AP Exam On the multiple-choice section, elongation usually shows up inside questions about the stages of translation and where energy is used. A classic stem describes a chemical that blocks peptide bond formation and asks which stage it hits. The answer is elongation, because that's where peptide bonds get made. Other stems compare translation energy use in prokaryotes versus eukaryotes, or test whether you can tell elongation apart from initiation (where the ribosome assembles at the start codon and the P site is set up). On FRQs you most often use elongation as part of a larger explanation, like tracing genotype to phenotype or describing why a mutation changes a protein. Note that the term "elongation" can also appear in a different context, as in the 2017 short FRQ about gibberellin promoting plant stem elongation, so read carefully to see whether the question means cell growth or the translation step. ## elongation vs initiation Initiation is the setup: the ribosome assembles on the mRNA at the start codon and the first tRNA lands in the P site. Elongation is the building phase that comes next, where the ribosome moves along and adds each new amino acid with a peptide bond. If a question is about peptide bonds forming repeatedly, that's elongation, not initiation. ## Key Takeaways - Elongation is the middle stage of translation, sandwiched between initiation and termination (EK 6.4.A.3). - During elongation the ribosome reads the mRNA one codon at a time and adds amino acids to the polypeptide using peptide bonds. - A chemical that blocks peptide bond formation acts during elongation, since that's when peptide bonds are made. - Elongation occurs on ribosomes in the cytoplasm of both prokaryotes and eukaryotes, and on the rough ER in eukaryotes (EK 6.4.A.1). - Elongation is how the genetic code becomes a real protein, which links genotype to phenotype under learning objective 6.4.A. ## FAQs ### What is elongation in translation? It's the stage where the ribosome moves along the mRNA codon by codon and adds amino acids one at a time to the growing polypeptide chain by forming peptide bonds. It comes after initiation and before termination. ### Is elongation when peptide bonds form? Yes. Peptide bonds linking each new amino acid to the chain are formed during elongation, which is why a chemical that blocks peptide bond formation has its main effect at this stage. ### How is elongation different from initiation? Initiation is the setup, where the ribosome assembles at the start codon and the first tRNA fills the P site. Elongation is the repeating build phase that adds each additional amino acid afterward. ### Does elongation differ between prokaryotes and eukaryotes? The core process is the same, but in prokaryotes elongation can begin while the mRNA is still being transcribed, since transcription and translation happen together (EK 6.4.A.2). In eukaryotes the mRNA is processed and exported first, so elongation happens later in the cytoplasm or on the rough ER. ### Is elongation on the AP Bio exam? Yes. It's part of topic 6.4 Translation in Unit 6 and shows up in multiple-choice questions about translation stages and energy use, and as a step you reference when explaining how genotype produces phenotype. ## 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/elongation#resource","name":"Elongation — AP Bio Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/elongation","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/elongation#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:36.675Z","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/elongation#term","name":"elongation","description":"In AP Bio, elongation is the second stage of translation, where the ribosome moves along the mRNA codon by codon and adds amino acids one at a time to the growing polypeptide chain via peptide bonds.","url":"https://fiveable.me/ap-bio/key-terms/elongation","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is elongation in translation?","acceptedAnswer":{"@type":"Answer","text":"It's the stage where the ribosome moves along the mRNA codon by codon and adds amino acids one at a time to the growing polypeptide chain by forming peptide bonds. 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