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Chapter 11 Meiosis and Sexual Reproduction Section 2 Meiosis Chapter 11 Section 2: Meiosis Key Vocabulary Terms Adapted from Holt Biology 2008 Meiosis A special process of cell division that results in haploid sex cells The total number of chromosomes decreases to half the original number Crossing-over The exchange of genetic material between homologous chromosomes during meiosis Independent Assortment The random distribution of pairs of genes on different chromosomes to the gametes Mitosis In eukaryotic cells, a process of cell division that forms two new nuclei, each of which has the same number of chromosomes Tetrad The four chromatids in a pair of homologous chromosomes that come together as a result of synapsis during meiosis Homologues Something homologous Genetic Variation Genetic diversity in a population or species as a result of new gene combinations (e.g. crossing over of chromosomes), genetic mutations, genetic drift, etc. Sperm The male gamete (sex cell) Ovum The female gamete: A mature egg cell Chapter 11 Section 2: Meiosis Notes Adapted from Holt Biology 2008 Stages of Meiosis During meiosis, a diploid cell divides twice to form four haploid cells. Meiosis I Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans). Meiosis I (PMAT1) • Prophase I – Homologous chromosomes pair – Chromatids exchange genetic material (crossing-over) • Metaphase I – Homologous pairs of chromosomes align at the cell equator • Anaphase I – Homologous pairs separate, chromatids do not. • Telophase I – two new haploid cells result (one set of chromosomes) Prophase I Chromosomes condense, and the nuclear envelope breaks down. Homologous chromosomes pair. Chromatids exchange genetic material. Metaphase I Homologous pairs of chromosomes align at the cell equator Anaphase I Homologous chromosomes separate. The spindle fibers pull the chromosomes of each pair to opposite poles of the cell without the chromatids separating at their centromeres. Each chromosome is still made of 2 chromatids. Telophase I The cytoplasm divides and two new cells are formed. Both cells have one chromosome from each pair of homologous chromosomes. Meiosis II (PMAT II) No more homologous chromosomes Meiosis II A separation of identical chromatids of each of the two haploid cells created in meiosis I, giving birth to four haploid cells. Prophase II The disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division Metaphase II The centromeres attach to spindle fibers from the centrosomes (centrioles) at each pole. Anaphase II The centromeres are cleaved, allowing microtubules to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. Telophase II Uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis II (PMAT II) • Prophase II – new spindle forms • Metaphase II – individual chromosomes align at the cell equator • Anaphase II – chromatids separate • Telophase II – four genetically different cells result 11-2 Meiosis (SKIP) • Compare Mitosis and Meiosis Mitosis Purpose Location Resulting cells Meiosis Genetic Variation Why is genetic variation in a population beneficial? (See the advantages of sexual reproduction on page 1.) Genetic Variation Three parts of sexual reproduction that contribute to variety: •Crossing-over •Independent assortment •Random fertilization Crossing-over • Occurs during prophase I of meiosis. • Sister chromatids of homologs exchange genetic material. • After crossing over, do sister chromatids on homologous chromosomes still have identical genetic material? Independent Assortment • Selection of one trait will not determine the selection of another. • Alignment of homologous chromosomes during metaphase I is random. • A pair of chromosomes can line up in two possible ways. • Resulting gametes are all different. Meiosis The red chromosomes don't influence the blue ones. Random Fertilization • The union of egg and sperm is random. • In humans, there are 70 trillion different possibilities. – 223 x 223 11-3 Life Cycles • Diploid Life Cycles – Diploid phase is dominant in humans Diploid Diploid Haploid Diploid 11-3 Life Cycles • Spermatogenesis – the formation of sperm 11-3 Life Cycles • Oogenesis – the formation of an ovum 11-3 Life Cycles • Haploid Life Cycles • – fungi: haploid phase is dominant 11-3 Life Cycles • Alternation of Generations – plants and multicellular protists