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Chapter 7 Meiosis Mitosis vs. Meiosis • What are the similarities and differences between mitosis and meiosis? – Similarities: • Nucleus divides • New cells are formed • Spindle is used – Differences: • • • • Meiosis = PMAT twice Crossing-over Mitosis forms 2 cells (diploid) Meiosis forms 4 cells (haploid) Objectives • Describe the similarities and differences between mitosis and meiosis. • Describe the differences and similarities between meiosis I and meiosis II. Meiosis • Cell division that halves the # of chromosomes when forming specialized reproductive cells, such as gametes – 8 steps (PMAT twice) • 2 Divisions of Nucleus: – Meiosis I – Meiosis II • Produces 4 haploid cells Prophase 1 • Chromosomes become visible • Nuclear envelope breaks down • Crossing-over occurs – Portions of chromatids are exchanged between chromosomes Metaphase 1 • Pairs of homologous chromosomes move to the equator of the cell. Anaphase 1 • Homologous chromosomes move to opposite poles Telophase 1 and Cytokinesis • Chromosomes gather at the poles of the cell • Cytoplasm divides • ***2 cells with 2 sets of chromosomes Prophase 2 • New spindle forms around the chromosomes Metaphase 2 • Chromosomes line up at the equator of the cell • Centromeres attached to spindle fibers Anaphase 2 • Centromeres divide • Chromatids move to opposite poles Telophase 2 and Cytokinesis • Nuclear envelope forms around each set of chromosomes • Cytoplasm divides • ***Forms 4 haploid cells Meiosis Flip Book • Using the small notepads, you will be creating a flip book of meiosis. – You may want to sketch in pencil before you begin!!! • Each of the 8 steps must be accurately drawn on separate pages. – ALSO…describe what is happening during each stage. • Be sure to begin your Flip Book with a title page that includes your name. Review • List the differences and similarities between meiosis 1 and meiosis 2. – Similarities: • Both go through PMAT • Nucleus divides – Differences: • Crossing-over occurs during Prophase 1 • 2 cells at the end of meiosis 1 • 4 cells at the end of meiosis 2 Objectives • Discuss the importance of independent assortment, crossing-over, and random fertilization to genetic variation. Meiosis and Genetic Variation • Meiosis creates new genetic variations • 3 mechanisms that create genetic variation: – Independent assortment – Crossing-over – Random fertilization Independent Assortment • Random distribution of homologous chromosomes during meiosis – Fig. 2 – p. 146 • Each of the 23 pairs of chromosomes separate independently • Creates 223 (8 million) different gene combinations Crossing-Over • Occurs when portions of chromatids are broken and exchange places – Occurs during Prophase I • Crossing-over and independent assortment create a nearly unlimited # of genetic combinations • Zygote forms from 2 gametes – 223 x 223 = 64 trillion genetic possibilities – Random Fertilization Crossing-Over Importance of Genetic Variation • Essential to evolution – Every organism of the same species would be the same • Speeds up evolution – Offspring not the exact same as parents Review • Take 5 minutes to write a short paragraph that discusses how independent assortment, crossing-over and random fertilization create new genetic variations. Objective • Compare the meiosis process in males and females. Meiosis and Gamete Formation • Meiosis occurs in all sexually reproducing organisms – May not be the same between males and females • Meiosis is the primary event that forms gametes – Gametogenesis Meiosis in Males • Spermatogenesis – Production of sperm • Sperm – Male gametes • Follows basic meiosis stages – p. 148 – fig. 3 Meiosis in Females • Oogensis – Production of eggs • Ovum – Female gametes • Cell division is not equal – p. 148 – fig. 3 Meiosis in Females • Cytoplasm divides unequally during meiosis 1 • One cell gets almost all of the cytoplasm – The other is called a polar body • Can divide again, but will not survive • Larger cell undergoes meiosis 2 – Unequal cell division again – Forms another polar body • Only one useable egg cell is produced Chapter 7 Quiz Activity • Write a quiz using the material we covered during Chapter 7. – Use the objectives to help you. • Your quiz must include: – 5 multiple choice questions – 5 matching questions – 1 short answer question • You must provide the correct answer for each question you create. Meiosis Simulation • Vocab. – Homologous Chromosomes • Pairs of chromosomes that are identical in size and appearance • Have the same genetic content – Tetrad • Formed when 2 homologous pairs of chromosomes are aligned side by side