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Chapter 13 Meiosis and Sexual Life Cycles PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Hereditary Similarity and Variation • Heredity is the transmission of traits from one generation to the next • Variation shows that offspring differ in appearance from parents and siblings • Genetics is the scientific study of heredity and variation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes • Genes are the units of heredity • Genes are segments of DNA • Locus – gene location on a chromosome • Reproductive cells - gametes (sperm and eggs) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction • Asexual reproduction, one parent produces genetically identical offspring by mitosis • Sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Bud Parent Video: Hydra Budding Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.5 mm Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 13-3 Karyotype Pair of homologous Chromosomes (homologues) Centromere Sister chromatids 5 µm • The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The number of chromosomes in a single set is represented by n • A cell with two sets is called diploid (2n) • For humans, the diploid number is 46 (2n = 46) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Key After DNA Synthesis Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) • Gametes are haploid cells, containing only one set of chromosomes • For humans, the haploid number is 23 (n = 23) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle Key • Gametes are the only types of human cells produced by meiosis, rather than mitosis • Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm cell (n) MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) LE 13-6 Key Haploid Diploid n Gametes n Mitosis n MEIOSIS Haploid multicellular organism (gametophyte) n FERTILIZATION Diploid multicellular organism Animals Zygote 2n Mitosis Mitosis Mitosis n n n Spores Gametes MEIOSIS 2n n Haploid multicellular organism n n n Gametes Diploid multicellular organism (sporophyte) n FERTILIZATION MEIOSIS 2n Mitosis n 2n Mitosis Plants and some algae Zygote FERTILIZATION 2n Zygote Most fungi and some protists Key Haploid Diploid Haploid multicellular organism (gametophyte) Mitosis n n Mitosis n n n Spores Gametes MEIOSIS 2n Diploid multicellular organism (sporophyte) FERTILIZATION 2n Mitosis Plants and some algae Zygote LE 13-6c Key Haploid Diploid Haploid multicellular organism Mitosis Mitosis n n n n Gametes MEIOSIS n FERTILIZATION 2n Zygote Most fungi and some protists Remember, only diploid cells can undergo meiosis. The Stages of Meiosis • In the first cell division (meiosis I), homologous chromosomes separate • Meiosis I results in two haploid daughter cells with replicated chromosomes • In the second cell division (meiosis II), sister chromatids separate • Meiosis II results in four haploid daughter cells with unreplicated chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 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 Meiosis I Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II Sister chromatids separate Haploid cells with unreplicated chromosomes LE 13-8aa INTERPHASE MEIOSIS I: Separates homologous chromosomes METAPHASE I Centrosomes (with centriole pairs) Chromatin Chromosomes duplicate Nuclear envelope ANAPHASE I • Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase I Terminology • Synapsis, homologous chromosomes loosely pair up, aligned gene by gene • Crossing over, non-sister chromatids exchange DNA segments • Tetrad, a group of four chromatids • Chiasmata, X-shaped regions where crossing over occurred Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 13-8ab MEIOSIS I: Separates homologous chromosomes METAPHASE I PROPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Metaphase plate Spindle Tetrad Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Microtubule attached to kinetochore Tetrads line up Homologous chromosomes separate Pairs of homologous chromosomes split up Telophase I and Cytokinesis • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells – In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animation: Telophase I and Cytokinesis LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II Cleavage furrow Two haploid cells form; chromosomes are still double METAPHASE II ANAPHASE II Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes LE 13-9 MITOSIS MEIOSIS Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Propase Prophase I Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Sister chromatids separate during anaphase 2n Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II n Property Mitosis Meiosis DNA replication Divisions During interphase One During interphase Two Synapsis and crossing over Daughter cells, genetic composition Do not occur Role in animal body Produces cells for growth and tissue repair Form tetrads in prophase I Four haploid, different from parent cell and each other Produces gametes Two diploid, identical to parent cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Sources of Genetic Variation/Diversity • Mutations (changes in an organism’s DNA) are the original source of genetic diversity • Reshuffling of different versions of genes during sexual reproduction produces genetic variation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins of Genetic Variation Among Offspring • Three mechanisms contribute to genetic variation in offspring: – Independent assortment of chromosomes – Crossing over – Random fertilization Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 13-10 Independent Assortment Key Maternal set of chromosomes Possibility 2 Possibility 1 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 LE 13-11 Nonsister chromatids Prophase I of meiosis Crossing Over Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) • The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations • Natural selection results in accumulation of genetic variations favored by the environment • Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings