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Meiosis and Sexual Life Cycles in Heredity and Variation

This chapter explores the process of meiosis and sexual reproduction, highlighting its role in heredity and variation in offspring. It discusses the stages of meiosis, the formation of gametes, and the differences between mitosis and meiosis. The concept of random fertilization and its contribution to genetic diversity is also explored.

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Meiosis and Sexual Life Cycles in Heredity and Variation

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  1. Chapter 13 Meiosis and Sexual Life Cycles

  2. Figure 13.1 • Heredity • Is the transmission of traits from one generation to the next • Variation • Shows that offspring differ somewhat in appearance from parents and siblings

  3. Parent Bud 0.5 mm Figure 13.2 Comparison of Asexual and Sexual Reproduction • In asexual reproduction • One parent produces genetically identical offspring by mitosis

  4. In sexual reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

  5. Meiosis • Definition – form of nuclear division that reduces the chrom # in half (2N  1N) • Occurs in eukaryotic organisms that reproduce sexually • Usually occurs to form GAMETES (sex cells) • Following fertilization, chromosome # is maintained

  6. Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) FERTILIZATION MEIOSIS Diploid zygote (2n = 46) Ovary Testis Mitosis and development Multicellular diploid adults (2n = 46) Figure 13.5 • The human life cycle

  7. Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n 2n Diploid multicellular organism Mitosis Figure 13.6 A (a) Animals • In animals • Meiosis occurs during gamete formation • Gametes are the only haploid cells

  8. Meiosis Steps There a two division in meiosis (I & II) • Meiosis I – separates HOMOLOGUES (pairs of chromosomes that are derived from each parent in the diploid organism • Meiosis II – separate sister chromatids (like mitosis but the end product is haploid)

  9. 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 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes The Stages of Meiosis • An overview of meiosis

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

  11. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate 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 Figure 13.8 • Telophase I, cytokinesis, and meiosis II

  12. 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. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Crossing Over • Crossing over • Produces recombinant chromosomes that carry genes derived from two different parents

  14. 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. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Figure 13.9 • A comparison of mitosis and meiosis

  16. Random Fertilization • The fusion of gametes • Will produce a zygote with any of about 64 trillion diploid combinations

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