1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 13 Meiosis and Sexual Life Cycles

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heredity – Is the transmission of traits from one generation to the next Variation – Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction In asexual reproduction – One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In sexual reproduction – Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis 1.Definition – form of nuclear division that reduces the chrom # in half (2N  1N) 2.Occurs in eukaryotic organisms that reproduce sexually 3.Usually occurs to form GAMETES (sex cells) 4.Following fertilization, chromosome # is maintained

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The human life cycle

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n Zygote Haploid Diploid Mitosis (a) Animals

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis Steps There a two division in meiosis (I & II) 1.Meiosis I – separates HOMOLOGUES (pairs of chromosomes that are derived from each parent in the diploid organism 2.Meiosis II – separate sister chromatids (like mitosis but the end product is haploid)

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase and meiosis I Figure 13.8

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase I, cytokinesis, and meiosis II

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Differences btwn Mitosis and Meiosis 1. Synapsis and crossing over in PROPHASE I – Homologous chromosomes physically connect and exchange genetic information 2. Tetrads on the metaphase plate – At metaphase I, paired homologous chromosomes (tetrads) line up

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Differences cont. 3. Separation of homologues – At anaphase I, homologous pairs move toward opposite poles of the cell – In anaphase II, the sister chromatids separate

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6 A comparison of mitosis and meiosis

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization The fusion of gametes – Will produce a zygote with any of about 64 trillion diploid combinations