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Meiosis Overview of Meiosis • Sexual reproduction • Gametes, with half the # of chromosomes, are produced • 2n1n • Ensures that the next generation will have a combination of traits from both parents Overview of Meiosis • Homologous chromosomes: similarly constructed chromosomes with the same shape and that contain genes for the same traits (homologues) • Just like in mitosis, meiosis occurs after interphase, when the cell grows, DNA is replicated, and chromosomes are duplicated. • Genetic recombination: production of offspring with combinations of traits that differ from those found in either parent. – Crossing over – Independent assortment Meiosis I • Prophase I – Synapsis occurs (pairing of homologous chromosomes) – Spindle appears – Nuclear membrane fragments – Nucleolus dissappears Meiosis I • Prophase I – Synapsis • Homologous chromosomes line up side by side – Crossing-over: exchange of genetic material between non-sister chromatids – Result= chromatids held together by centromere are no longer identical – Daughter cells will receive chromosomes with recombined genetic material Meiosis I • Metaphase I – Homologous pairs line up at equator • Maternal and paternal member may be oriented toward either pole. • In humans, results in 223 (8,388,608) combinations of maternal and paternal chromosomes in gametes • Anaphase I – Homologous chromosomes separate, pulled to opposite poles by spindle fibers – Independent assortment: all possible combinations of chromosomes occur in the daughter nuclei. • Telophase I – Nuclear membrane reforms – Nucleoli appear – Cytokinesis produces two haploid daughter cells • Interkinesis: between meiosis I and II – No DNA replication since chromosomes are already dublicated • Prophase II: spindle appears, nuclear membrane disassembles, nucleolus disappears • Metaphase II: chromosomes attach to spindles and align at equator • Anaphase II: sister chromatids separate becoming daughter chromosomes • Telophase II: spindle disappears, nuclei form, nuclear membrane reforms • Cytokinesis occurs • Meiosis results in four haploid daughter cells • Nondisjunction – Failure of homologous chromosomes to separate during anaphase I, or sister chromatids to separate at anaphase II – Results in gametes with abnormal number of chromosomes – Causes various syndromes • Trisomy 21 (Down Syndrome)—extra copy of chromosome 21 • Turner Syndrome—absence of second X chromosome • Daughter cells mature into gametes (eggs, sperms) that fuse during fertilization – Restores the diploid number of chromosomes in a cell that will develop into a new individual.