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Transcript
Meiosis Notes CP Biology Ms. Morrison Chromosome Number Every organism gets half of chromosomes from one parent and half from the other parent Two sets of chromosomes = homologous chromosomes Chromosome Number, pg 2 Cells with homologous chromosomes = diploid (2N) Gametes (egg, sperm) – contain only one set of chromosomes = haploid (N) Humans – 23 pairs (46 chromsomes) Meiosis Form of cell division to reduce the number of chromosomes in cell to half (2N N) Two stages to process: Meiosis I and Meiosis II At end of Meiosis II – one diploid cell has become four haploid cells Used to produce gametes (does not occur in regular body cells) Meiosis I Picture overview: Meiosis I, pg 2 Interphase I – all chromosomes are replicated Prophase I – each chromosome pairs with its homologous chromosome to form a tetrad (4 chromatids) Meiosis I, pg 3 Prophase I – during this phase, chromosomes in tetrads may exchange portions of their chromatids in process = crossing over (gives new gene combinations Meiosis I, pg 4 Metaphase I – spindle fibers attach to chromosomes along center of cell Anaphase I – spindle fibers pull the homologous chromosomes toward opposite ends of the cell Meiosis I, pg 5 Telophase I/Cytokinesis – nuclear membranes form and the cell separates into two cells At end of Meiosis I – two daughter cells that are haploid because they contain one set of chromosomes (duplicated, though) Meiosis II Two haploid daugther cells go through another division similar to mitosis but does NOT have replication prior to prophase II Meiosis II, pg 2 Prophase II – nuclear membranes disappear and spindle forms Metaphase II – chromosomes line up along center of cell like those in mitosis Meiosis II, pg 3 Anaphase II – sister chromatids separate and move towards opposite sides of the cell Telophase II/Cytokinesis – nuclear membranes reform and the two cells separate into four cells – all are haploid Gamete Formation Males – haploid gametes = sperm, always produce four sperm through meiosis Females – haploid gamete = egg, uneven divisions through Meiosis I & II so egg receives most of cytoplasm, remaining three cells = polar bodies (not used in reproduction)