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Meiosis Reduction Division • In sexual reproduction, gametes fuse (fertilization) to produce a zygote Meiosis reduces the number of chromosomes to half that found in other body cells • Adult somatic cells are diploid, 2n • Gamete cells are haploid, n Meiosis: Two Divisions • Two consecutive nuclear divisions – Meiosis I – Meiosis II • DNA is not duplicated between divisions • Four haploid nuclei are formed Meiosis I Each homologue in the cell pairs with its partner, then the partners separate Meiosis II • The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) Stages of Meiosis Meiosis I Meiosis II • Prophase I • Metaphase I • Prophase II • Metaphase II • Anaphase I • Anaphase II • Telophase I • Telophase II Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I Prophase I • Each duplicated, condensed chromosome pairs with its homologue • Homologues swap segments= crossing over • Each chromosome becomes attached to microtubules of newly forming spindle Metaphase I • Chromosomes move to the middle of cell • Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole • The spindle is now fully formed Anaphase I • Homologous chromosomes segregate from each other • The sister chromatids of each chromosome remain attached Telophase I • The chromosomes arrive at opposite poles • The cytoplasm divides • There are now two haploid cells • This completes Meiosis I Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II Prophase II • Microtubules attach to the duplicated chromosomes • Chromosomes move to the spindle’s equator Metaphase II • All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles Anaphase II • Sister chromatids separate to become independent chromosomes • Chromosomes move to opposite poles Telophase II • The chromosomes arrive at opposite ends of the cell • A nuclear envelope forms around each set of chromosomes • The cytoplasm divides • There are now four haploid cells Stages of Meiosis Meiosis I Meiosis II • Prophase I • Metaphase I • Prophase II • Metaphase II • Anaphase I • Anaphase II • Telophase I • Telophase II
