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Genetics 3.3 Meiosis 10.1 Meiosis (AHL) Recall Mitosis • Purpose: • Process: • Results: Sexual Reproduction • • • • What is sexual reproduction? Why is it beneficial? What kinds of cells are involved? How are these cells formed? Diploid vs Haploid • Diploid nuclei have pairs of homologous chromosomes. • Haploid nuclei have one chromosome of each pair. Meiosis • Meiosis is the process by which sex cells, or gametes, are formed. ▫ In humans this takes place in the testes and ovaries • Meiosis involves 2 stages of cell division. • In the end, daughter cells contain half the chromosome number of the parent cell = haploid. • Halving of the chromosome number allows a sexual life cycle with fusion of the gametes. Stages of Meiosis: (Make a table as shown on the board) • Meiosis I: ▫ Purpose: Reduce chromosome number ▫ Process: Prophase I, Metaphase I, Anaphase I, Telophase I ▫ Results: Two Daughter cells each containing half the number of chromosomes (still in the form of sister chromatids). • Meiosis II: ▫ Purpose: Separate sister chromatids ▫ Process: Prophase II, Metaphase II, Anaphase II, Telophase II ▫ Results: Total of Four haploid Daughter cells (which may become gametes). Crash Course Biology Video: https://www.youtube.com/watch?v=qCLmR9-YY7o Meiosis I • Prophase I: ▫ ▫ ▫ ▫ Nuclear membrane dissolves Centrioles split and move to opposite poles Spindle fibers form Chromosomes come together in homologous pairs Each chromosome is composed of a pair of sister chromatids The whole structure is referred to as a tetrad because it contains 4 chromatids ▫ The intertwined chromatids may break and exchange segments of genetic material = crossing over Meiosis I • Metaphase I: ▫ Homologous chromosomes attach to spindle fibers ▫ In pairs, they line up along the equatorial plate Meiosis I • Anaphase I: ▫ “Reduction division” occurs ▫ One member of each Homologous pair moves towards either pole. Each chromosome still contains sister chromatids Meiosis I • Telophase I: ▫ Membranes form to create 2 new nuclei ▫ Each of the daughter nuclei contains one member of each chromosome pair. ▫ Telophase is followed by cytokinesis, the division of the cytoplasm. Meiosis II • Prophase II: ▫ Nuclear membrane dissolves ▫ Spindle fibers form Meiosis II • Metaphase II: ▫ Chromosomes, each with two chromatids, line up along the equatorial plate. Meiosis II • Anaphase II: ▫ Attachment between the sister chromatids is broken ▫ Individual chromatids move to opposite poles Meiosis II • Telophase II: ▫ Nuclear membranes form ▫ Form haploid nuclei have been produced ▫ Telophase is followed by cytokinesis. Meiosis • Complete the worksheets Meiosis • Complete the practical activity ▫ P 110 and 111 in Bio 11 textbook Bivalent formation and crossing over • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. • Bivalent = a pair of homologous chromosomes • Synapsis = pairing process • Junction = the area where chromosomes break and rejoin • Chiasmata = the point of connection Random Orientation of the bivalents • Orientation of pairs of homologous chromosomes prior to separation is random. • Orientation = the pole to which each chromosome is attached depends on the way it is facing. • For every additional bivalent, the number of possible chromosome combinations in a cell produced by meiosis doubles (2n). ▫ Calculate the number of possible combinations in humans Non-disjunction • Non-disjunction = when homologous chromosomes fail to separate in anaphase. • Both of the chromosomes move to one of the poles. • The result is a gamete with either an extra chromosome or deficient in a chromosome. • If this gamete goes on to be fertilized, the result will be an individual with 45 or 47 chromosomes. • Example: Down Syndrome