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

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 Video: Hydra Budding Video: Hydra Budding Parent 0.5 mm Bud

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE µm Pair of homologous Chromosomes (homologues) Sister chromatids Centromere Karyotype

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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)

Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere After DNA Synthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm cell (n) Testis Ovary Mitosis and development Multicellular diploid adults (2n = 46) FERTILIZATIONMEIOSIS Diploid zygote (2n = 46)

LE 13-6 Key Haploid Diploid Gametes n Diploid multicellular organism (sporophyte) Mitosis Diploid multicellular organism FERTILIZATION MEIOSIS Zygote n n 2n2n 2n2n Animals Plants and some algae Most fungi and some protists n n n n n n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n 2n2n 2n2n Spores Haploid multicellular organism (gametophyte) Haploid multicellular organism

Key Haploid Diploid multicellular organism (sporophyte) Plants and some algae n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n 2n2n Spores Haploid multicellular organism (gametophyte)

LE 13-6c Key Haploid Diploid Most fungi and some protists n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n Haploid multicellular organism Remember, only diploid cells can undergo meiosis.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE 13-7 Homologous pair of chromosomes in diploid parent cell Interphase Homologous pair of replicated chromosomes Chromosomes replicate Meiosis I Diploid cell with replicated chromosomes Sister chromatids Meiosis II Homologous chromosomes separate Sister chromatids separate Haploid cells with replicated chromosomes Haploid cells with unreplicated chromosomes

LE 13-8aa Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate INTERPHASE MEIOSIS I: Separates homologous chromosomes METAPHASE I ANAPHASE I

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Animation: Telophase I and Cytokinesis Animation: Telophase I and Cytokinesis

LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

LE 13-9 Propase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Parent cell (before chromosome replication) Chromosome replication MITOSISMEIOSIS Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Metaphase I Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n = 3 Anaphase I Telophase I MEIOSIS II Daughter cells of mitosis 2n2n 2n2n n Sister chromatids separate during anaphase II n nn Daughter cells of meiosis II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PropertyMitosisMeiosis DNA replication During interphase DivisionsOneTwo Synapsis and crossing over Do not occurForm tetrads in prophase I Daughter cells, genetic composition Two diploid, identical to parent cell Four haploid, different from parent cell and each other Role in animal body Produces cells for growth and tissue repair Produces gametes

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

LE Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Combination 2 Combination 1 Combination 3 Combination 4 Daughter cells Metaphase II Two equally probable arrangements of chromosomes at metaphase I Independent Assortment

LE Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells Crossing Over

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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