Chapter 12 - MeiosisIntroductionChromosomes Come in Distinct TypesSlide 4Homologous Chromosomes Have the Same GenesSlide 6The Concept of PloidyPloidy Varies among OrganismsSlide 9An Overview of MeiosisSlide 11Meiosis Is Two Cell DivisionsAn Overview of Meiosis IAn Overview of Meiosis IISlide 15Meiosis Is a Reduction DivisionFertilization Results in a Diploid ZygoteSlide 18The Life Cycle of a Sexual OrganismSlide 20The Phases of Meiosis ISlide 22Slide 23Slide 24Slide 25The Result of Meiosis IThe Phases of Meiosis IISlide 28The Result of Meiosis II***Comparison of Meiosis and Mitosis***Slide 31Slide 32Where Does Crossing Over Occur?A Closer Look at Crossing OverSlide 35The Consequences of MeiosisSexual Reproduction Leads to Greater VariationIndependent Assortment Produces Genetic VariationSlide 39The Role of Crossing OverHow Does Fertilization Affect Genetic Variation?Slide 42Outcrossing Further Increases Genetic VariationWhy Does Meiosis Exist?The Paradox of SexSlide 46The Purifying Selection HypothesisThe Changing Environment HypothesisTesting the Changing Environment HypothesisMistakes in MeiosisHow Do Mistakes Occur?Slide 52Types of NondisjunctionFrequency of NondisjunctionWhy Do Mistakes Occur?Slide 56Most Common Aneuploidy DisordersKey ConceptsSlide 59© 2011 Pearson Education, Inc.Chapter 12 - Meiosis© 2011 Pearson Education, Inc.Introduction•During sexual reproduction, a sperm and an egg unite to form a new individual.–This process is called fertilization. •Meiosis is nuclear division that precedes the formation of gametes (egg and sperm) and results in a halving of chromosome number.© 2011 Pearson Education, Inc.Chromosomes Come in Distinct Types•Each organism has a characteristic number of chromosomes.•The karyotype is the number and types of chromosomes present in an organism.•Sex chromosomes determine the sex of the individual; all other chromosomes are autosomes.–Humans have 46 chromosomes in every cell except their gametes.–1 pair of sex chromosomes.–22 pairs of autosomes.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Homologous Chromosomes Have the Same Genes•Chromosomes of the same type are called homologous chromosomes, or homologs.•Chromosomes carry genes. A gene is a section of DNA that influences one or more hereditary traits in an individual. –Different versions of a specific gene are called alleles.•Homologs carry the same genes in the same locations, but each one may contain different alleles.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Concept of Ploidy•The haploid number n indicates the number of distinct types of chromosomes present. •A cell’s ploidy (n, 2n, 3n, etc.) indicates the number of each type of chromosome present.© 2011 Pearson Education, Inc.Ploidy Varies among Organisms •Organisms whose cells contain just one of each type of chromosome are called haploid (n).•Those whose cells contain two versions of each type of chromosome are termed diploid (2n).–Diploid cells have one paternal chromosome and one maternal chromosome.•Organisms with three or more versions of each type of chromosome are called polyploid (3n, 4n, etc.)© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.An Overview of Meiosis•Meiosis reduces chromosome number by half. In diploid organisms, the products of meiosis are haploid.•Just before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated. –When replication is complete, each chromosome consists of two identical sister chromatids attached at the centromere.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Meiosis Is Two Cell Divisions•Meiosis consists of two cell divisions, called meiosis I and meiosis II. •The two divisions occur consecutively but differ sharply.© 2011 Pearson Education, Inc.An Overview of Meiosis I•During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells.•The homologs in each chromosome pair separate and go to different daughter cells. •Although the daughter cells are haploid (n), each chromosome still consists of two identical sister chromatids.© 2011 Pearson Education, Inc.An Overview of Meiosis II•During meiosis II, the sister chromatids of each chromosome separate and go to different daughter cells. •The four haploid daughter cells produced by meiosis II also have one of each type of chromosome, but now the chromosomes are unreplicated.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Meiosis Is a Reduction DivisionThe outcome of meiosis is a reduction in chromosome number. For this reason, meiosis is known as a reduction division.•In most plants and animals, the original cell is diploid and the four daughter cells are haploid.–In animals, these daughter cells become gametes via a process called gametogenesis.© 2011 Pearson Education, Inc.Fertilization Results in a Diploid Zygote•When two haploid gametes fuse during fertilization, a full complement of chromosomes is restored. The cell that results from fertilization is diploid and is called a zygote.•In this way, each diploid individual receives a haploid chromosome set from both its mother and its father. –Homologs are therefore referred to as being either maternal chromosomes, from the mother, or paternal chromosomes, from the father.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Life Cycle of a Sexual Organism •An animal’s life cycle summarizes life from fertilization through offspring production.•Meiosis in an adult produces haploid gametes that combine during fertilization to form a diploid zygote, which develops, through mitosis, into an adult of the next generation.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Phases of Meiosis I•Meiosis I is a continuous process with five distinct phases. These phases are as follows:1. Early prophase I2. Late prophase I3. Metaphase I4. Anaphase I5. Telophase I© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Phases of Meiosis I•Early Prophase I: The homolog pairs come together in a pairing process called synapsis. The structure that results from synapsis is called a tetrad, consisting of two homologs. The chromatids of the homologs are called non-sister chromatids.•Late Prophase I: These non-sister chromatids begin to separate. Exchange or crossing over between homologous non-sister chromatids occurs where chiasmata are