Meiosis and Sexual Reproduction. Chapter 10. Halving the Chromosome Number. Homologous Pairs

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1 Meiosis and Sexual Reproduction Chapter 10 Outline Reduction in Chromosome Number Homologous Pairs Meiosis Overview Genetic Recombination Crossing-Over Independent Assortment Fertilization Meiosis I Meiosis II Human Life Cycle 1 2 Halving the Chromosome Number In sexually reproducing organisms, meiosis reduces the chromosome number from diploid to haploid. Gametes are haploid, and the fusion of two gametes forms a diploid organism. Homologous Pairs Homologous chromosomes (homologues) are paired chromosomes that code for the same traits (one inherited from each parent). 3 4

2 Overview of Meiosis Meiosis requires two nuclear divisions and produces four haploid daughter cells. Daughter cells receive one of each kind of parental chromosome, but in different combinations. Synapsis results in a bivalent.! Homologues align and separate. Overview of Meiosis Daughter chromosomes, derived from sister chromatids, move to opposite poles. Chromosomes in the four daughter cells have only one chromatid.! Meiosis II " Meiosis I 5 6 Meiosis Overview Meiosis Overview 7 8

3 Genetic Recombination Meiosis helps ensure genetic recombination. In a changing environment, asexual reproduction might saddle an offspring with a parent s disadvantageous gene combination. Sexual reproduction might give offspring better chance of survival. Genetic Recombination - Crossing-over is an exchange of genetic material between nonsister chromatids of a bivalent during meiosis I.! Homologues are held together by chiasmata, regions where the nonsister chromatids are attached due to crossover Fig Crossing-Over 11 12

4 Genetic Recombination Independent Assortment Independent assortment allows homologous chromosomes to separate independently.! When homologues align at the equator, maternal or paternal homologue may be oriented toward either pole Genetic Recombination Phases of Meiosis Fertilization! Gametes produced by one person are expected to be genetically different from the gametes produced by another person. " Fusion of gametes causes combination of parental genes. # (2 23 ) 2 combinations Prophase I Homologous chromosomes pair during synapsis. Metaphase I Homologous pairs align at the metaphase plate

5 Meiosis I Phases of Meiosis Anaphase I Homologous chromosomes separate, and move towards poles. Telophase I Daughter cells have one chromosome from each homologous pair. Interkinesis Chromosomes still contain two chromatids Fig Meiosis I 19 20

6 Meiosis II Meiosis II Duplicated, haploid chromosomes align at metaphase plate. Sister chromatids separate at the centromere, giving rise to daughter chromosomes.! At end of telophase II and cytokinesis, there are four haploid cells Fig Meiosis II 23 24

7 Meiosis vs. Mitosis Meiosis vs. Mitosis Fig Table

8 Table 10.2 Human Life Cycle In animals, meiosis occurs only during gamete production. In males, meiosis is a part of spermatogenesis. In females, meiosis is a part of oogenesis Spermatogenesis Oogenesis 31 32

9 Review Reduction in Chromosome Number Homologous Pairs Meiosis Overview Genetic Recombination Crossing-Over Independent Assortment Fertilization Meiosis I Meiosis II Human Life Cycle 33 34