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Meiosis (Gamete Formation) Meiosis – cell division that creates gametes (sex cells, sperm and egg) with half the number of chromosomes as the parent body cell 2 cell divisions – Meiosis I and Meiosis II 1. Interphase (just like mitosis) 2. Meiosis I a. Prophase I nucleus disappears homologous chromosomes pair up to form a tetrad crossing over occurs b. Metaphase I pairs of homologous chromosomes line up in the middle of the cell random assortment occurs c. Anaphase I pairs of homologous chromosomes separate and move to opposite ends of the cell d. Telophase I and Cytokinesis chromosomes uncoil nuclei reappear cytoplasm divides 3. Meiosis II a. Prophase II through Telophase II JUST LIKE MITOSIS! Meiosis # of cells # of chromosomes/cell Before Meiosis After Meiosis http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://biology.about.com/od/meiosis/ss/meiosisstep.htm http://www3.moe.edu.sg/edsoftware/ir/files/biomeiosis/stages_meiosis1/stages_meiosis1_frameset.html In humans… Why are you “you”nique? - Meiosis accounts for why we’re all different! (except for identical twins) - Sources of variation provided by meiosis (when YOU are making sperm/egg): 1. During Interphase, DNA can change (mutation) during DNA replication. 2. During Prophase I, crossing over causes chromosomes to exchange DNA and shuffle your mom and dad’s genes. 3. During Metaphase I, your mom and dad’s DNA line up in different ways each time you make gametes (random assortment). 4. During Metaphase II, occasionally chromosomes do not separate like they should (nondisjunction) leading to conditions such as Down syndrome. 5. During fertilization, one unique sperm meets one unique egg. With ALL that variation, you make 8 million different sperm or egg. This means you your spouse can make a total of 70 trillion different children!!