--- title: "Anaphase I — AP Biology Definition & Exam Guide" description: "Anaphase I is the meiosis I stage where homologous chromosomes separate to opposite poles, setting up haploid gametes and driving genetic diversity in AP Bio Unit 5." canonical: "https://fiveable.me/ap-bio/key-terms/anaphase-i" type: "key-term" subject: "AP Biology" unit: "Unit 5" --- # Anaphase I — AP Biology Definition & Exam Guide ## Definition Anaphase I is the stage of meiosis I when homologous chromosomes (not sister chromatids) separate and move to opposite poles of the cell. Sister chromatids stay attached at their centromeres, which is what separates meiosis from mitosis here. ## What It Is Anaphase I is the stage of [meiosis I](/ap-bio/key-terms/meiosis-i "fv-autolink") where the two homologous chromosomes in each pair get pulled apart, one heading to each pole of the cell. The key detail: the **sister chromatids stay stuck together** at their centromeres. Only the homologs separate. This is different from mitosis, where sister chromatids split. Think of it this way. Coming out of metaphase I, homologous pairs are lined up at the [metaphase plate](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z "fv-autolink") (EK 5.1.A.2). [Spindle fibers](/ap-bio/unit-4/homeostasis-feedback-loops/study-guide/OgMSpwCrEez0qyWtlCAC "fv-autolink") then drag each whole chromosome (still two chromatids) to opposite poles. Because each pole gets one chromosome from each homologous pair, the cell is now set up to become **haploid (1n)** after the rest of meiosis finishes (EK 5.2.A.1). Proper separation here is what guarantees each future gamete ends up with one full set of chromosomes. ## Why It Matters Anaphase I lives in **[Unit 5](/ap-bio/unit-5 "fv-autolink"): Heredity**, under topics 5.1 (Meiosis) and 5.2 (Meiosis and Genetic Diversity). It directly supports **[AP Bio](/ap-bio "fv-autolink") 5.1.A** (how meiosis transmits chromosomes to the next generation) and **AP Bio 5.2.A** (how meiosis generates genetic diversity). The separation of homologs at anaphase I is the physical event behind Mendel's law of independent assortment, since which homolog goes to which pole is random for every pair. It also underpins **AP Bio 5.1.B**, the compare-and-contrast of mitosis versus meiosis, because separating homologs (not chromatids) is the defining difference at this stage. ## Connections ### Disjunction and Nondisjunction (Unit 5) Anaphase I is exactly when disjunction (correct separation of homologs) happens. If it fails, that's [nondisjunction](/ap-bio/key-terms/nondisjunction "fv-autolink"), and gametes end up with the wrong chromosome number instead of being cleanly haploid (EK 5.2.A.1). ### [Sister Chromatids (Unit 5)](/ap-bio/key-terms/sister-chromatids) [Sister chromatids](/ap-bio/key-terms/sister-chromatids "fv-autolink") stay glued together through anaphase I. They don't split until anaphase II. Knowing this is the fastest way to tell which anaphase you're looking at. ### [Mendel's Law of Independent Assortment (Units 5, 6)](/ap-bio/key-terms/mendels-law-of-independent-assortment) The random way each homologous pair separates at anaphase I is the cellular basis for [independent assortment](/ap-bio/key-terms/independent-assortment "fv-autolink"). It links the physical event you see under a microscope to the inheritance ratios you predict in genetics problems. ### Genetic Diversity and Gametes (Unit 5) Random orientation of homologs at metaphase I, resolved by their separation at anaphase I, shuffles maternal and paternal chromosomes into each gamete, boosting genetic diversity alongside crossing over (EK 5.2.A.2). ## On the AP Exam MCQ stems love to test whether you can identify a stage from a description. A classic version says a cell has homologous chromosomes moving to opposite poles while sister chromatids stay attached at their centromeres, and asks for the stage. The answer is anaphase I, and the giveaway is that the chromatids haven't split. You'll also see questions on what happens when separation fails: a 2025 Short FRQ centered on the ALD protein, which attaches to centromeres and spindle filaments during meiosis, and asked you to reason about what goes wrong without it. Be ready to explain that failed separation at anaphase I causes nondisjunction, producing gametes that aren't haploid and disrupting chromosome transmission to the next generation. ## Anaphase I vs Anaphase II Anaphase I separates homologous chromosomes (sister chromatids stay together). Anaphase II separates sister chromatids, just like mitosis does. If chromatids are splitting, you're in anaphase II; if whole homologs are pulling apart with chromatids intact, you're in anaphase I. ## Key Takeaways - In anaphase I, homologous chromosomes separate and move to opposite poles, but sister chromatids stay attached at their centromeres. - This sets up the haploid (1n) gamete because each pole receives one chromosome from each homologous pair. - Separating homologs (not chromatids) is the key difference between anaphase I of meiosis and anaphase of mitosis. - Failure to separate at anaphase I is nondisjunction, which produces gametes with the wrong chromosome number. - The random orientation of pairs resolved at anaphase I is the physical basis for Mendel's law of independent assortment. ## FAQs ### What happens during anaphase I of meiosis? Homologous chromosomes are pulled apart and move to opposite poles of the cell. Each chromosome still has both sister chromatids attached at the centromere, since chromatids don't separate until anaphase II. ### Do sister chromatids separate in anaphase I? No. Sister chromatids stay attached during anaphase I; only homologous chromosomes separate. Sister chromatids don't split until anaphase II. This is the most-tested distinction in Unit 5. ### How is anaphase I different from anaphase II? Anaphase I separates homologous chromosomes while keeping sister chromatids together. Anaphase II separates the sister chromatids, exactly like mitosis. The number of attached chromatids is your clue to which stage you're seeing. ### What happens if separation fails during anaphase I? That's nondisjunction. Homologs that fail to separate end up at the same pole, so the resulting gametes are no longer haploid and carry the wrong number of chromosomes (EK 5.2.A.1), disrupting transmission to the next generation. ### How does anaphase I connect to genetic diversity? The orientation of each homologous pair at the metaphase plate is random, and anaphase I resolves that into different combinations of maternal and paternal chromosomes per pole. This is the cellular basis for independent assortment and adds diversity on top of crossing over. ## Related Study Guides - [5.1 Meiosis](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z) - [5.2 Meiosis and Genetic Diversity](/ap-bio/unit-5/meiosis-genetic-diversity/study-guide/YZOFYsQw4twZNkp5WM3l) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-bio/key-terms/anaphase-i#resource","name":"Anaphase I — AP Biology Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/anaphase-i","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/anaphase-i#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T00:49:58.524Z","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/anaphase-i#term","name":"Anaphase I","description":"Anaphase I is the stage of meiosis I when homologous chromosomes (not sister chromatids) separate and move to opposite poles of the cell. 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