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Meiosis and Binary Fission Notes January 1920, 2012 1/19 (R) Meiosis (textbook pages 270276) Purpose : produce haploid gametes Chromosomes/Chromosome Number • humans have 46 chromosomes (23 pairs) • Gene DNA segments carry instructions for traits (characteristics) and cell function Pair of Homologous Chromosomes same location of genes for specific traits centromere male female Haploid and Diploid Cells • Gamete sex cells with 1/2 number of chromosomes • Haploid cell with n # of chromosomes • Diploid cell with 2n # of chromosomes (n from male, n from female) • Fertilization 1 haploid gamete combines with another haploid gamete to form a diploid organism Meiosis and Binary Fission Notes January 1920, 2012 MEIOSIS I • produces gametes • reduces # of chromosomes **mitosis keeps same # chromosomes **meiosis reduces # chromosomes by 1/2 Stages of meiosis I • Interphase • Prophase I • Metaphase I • Anaphase I • Telophase I • Cytokinesis Interphase • DNA replicates • protein synthesis Prophase I • replicated chromosomes visible sister chromatids • homologous chromosomes condnese to for pairs in process called synapsis centromere sister chromatids Meiosis and Binary Fission Notes January 1920, 2012 **Crossing over chromosome segments exchange between homologous chromosome pairs • centrioles move to opposite sides (poles) • spindle fibers form, bind to sister chromatids at centromere Metaphase I • homologous chromosome pairs line up at equator **mitosis individual chromosomes line up **meiosis paired chromosomes line up Meiosis and Binary Fission Notes January 1920, 2012 1/20 (F) Anaphase I • chromosomes separate • chromosome # reduces from 2n to n **mitosis sister chromatids split **meiosis homologous chromosome pairs split (still have 2 sister chromatids) Telophase I • poles contain 1/2 of chromosome pair **sister chromatids NOT identical because of crossing over Cytokinesis • cytoplasm splits • cells go to Meiosis II **meiosis only HALF completed **have 2 diploid cells which are not genetically identical MEIOSIS II Interphase • DNA does NOT replicate Prophase II • chromosomes condense • spindle fibers form Meiosis and Binary Fission Notes January 1920, 2012 Metaphase II • diploid chromosomes line up at equator **mitosis haploid chromosomes line up Anaphase II • sister chromatids pulled apart, move to poles Telophase II • chromosomes reach poles (haploid) • nuclei & nuclear membrane reappear Cytokinesis • cytoplasm divides **Mitosis produces 2 genetically identical diploid cells **Meiosis produces 4 genetically DIFFERENT haploid cells Genetic Variation occurs during : meiosis crossing over fertilization Meiosis and Binary Fission Notes January 1920, 2012 Binary Fission (textbook page 520) • division of a cell into 2 genetically identical cells • chromosomes replicate • new copies separate • cell elongates (gets longer) • new plasma membrane and cell wall form to separate cell into 2 identical cells • 1 bacterium can become 1 billion bacteria in 10 hours (in ideal situation) chromosomes cytoplasm cell plasma wall membrane