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Slide 1 Chapter 13: Meiosis and Sexual Life Cycles Figure 13.1 Slide 3 The Birds and the Bees Asexual vs. Sexual reproduction Figure 13.2 0.5 mm Parent Bud (a) Hydra (b) Redwoods Slide 5 Slide 6 Chromosome arrangement • Somatic cells – typical body cells • 46 chromosomes in 23 pairs (humans) • The chromosomes are not normally paired up • Each pair is called homologous chromosomes • Gametes – sex cells • 23 chromosomes (humans) • One member from each pair Slide 7 Homologous chromosomes • 22 of the pairs (autosomes) are “true homologues” • One of each came from mom and dad • Identical in length and type of genes carried • Genes on each are slightly different • Sex chromosomes (23rd pair) don’t match up exactly (X vs Y) Figure 13.3a Figure 13.3b Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome 5 m Figure 13.4 Key 2n 6 Maternal set of chromosomes (n 3) Paternal set of chromosomes (n 3) Sister chromatids of one duplicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) Figure 13.5 Haploid gametes (n 23) Key Haploid (n) Diploid (2n) Egg (n) Sperm (n) The Human Life Cycle MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n 46) Mitosis and development Multicellular diploid adults (2n 46) Figure 13.6 Sexual Life Cycles Key Haploid (n) Diploid (2n) n Gametes n Mitosis n n MEIOSIS FERTILIZATION n Diploid multicellular organism Zygote 2n Mitosis (a) Animals (Including Humans) n Mitosis n Mitosis n Spores Gametes MEIOSIS 2n Haploid unicellular or multicellular organism Haploid multicellular organism (gametophyte) n n n n Gametes FERTILIZATION 2n Zygote Mitosis (b) Plants and some algae n FERTILIZATION MEIOSIS 2n Diploid multicellular organism (sporophyte) Mitosis 2n Zygote (c) Most fungi and some protists Slide 13 Meiosis • Reduces chromsome number • Produces haploid cells • Results in four daughter cells • All are haploid • All are genetically unique from each other and from the parent • DNA replicated before meiosis begins • Two stages • Meiosis I: separates homologous chromosomes • Meiosis II: separates sister chromatids Slide 14 How does meiosis reduce chromosome number? 2 divisions Figure 13.7-1 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Figure 13.7-2 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Figure 13.7-3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes Figure 13.8 MEIOSIS I: Separates sister chromatids MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Telophase I and Cytokinesis Anaphase I Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Anaphase II Telophase II and Cytokinesis Centromere (with kinetochore) Metaphase plate During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Cleavage furrow Fragments of nuclear envelope Metaphase II Sister chromatids remain attached Spindle Homologous chromosomes Prophase II Homologous chromosomes separate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Each pair of homologous chromosomes separates. Sister chromatids separate Two haploid cells form; each chromosome still consists of two sister chromatids. Haploid daughter cells forming Figure 13.8a Prophase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Telophase I and Cytokinesis Anaphase I Metaphase I Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Cleavage furrow Each pair of homologous chromosomes separates. Chromosomes line up Duplicated homologous chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2n 6 in this example. Two haploid cells form; each chromosome still consists of two sister chromatids. Figure 13.8b Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming Figure 13.9a MEIOSIS MITOSIS Parent cell MEIOSIS I Chiasma Prophase Prophase I Duplicated chromosome Chromosome duplication 2n 6 Chromosome duplication Homologous chromosome pair Metaphase Metaphase I Anaphase Telophase Anaphase I Telophase I Daughter cells of meiosis I 2n Daughter cells of mitosis Haploid n3 MEIOSIS II 2n n n n n Daughter cells of meiosis II Figure 13.9b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes Slide 23 Sexual Reproduction leads to genetic variation! • 3 processes that contribute to variation 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization Figure 13.10-1 1. Independent Assortment Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Each homologous pair lines up independently of the other pairs at the Metaphase Plate. Figure 13.10-2 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10-3 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Slide 27 2. Crossing Over Figure 13.11-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Figure 13.11-2 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Nonsister chromatids held together during synapsis Figure 13.11-3 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase I Nonsister chromatids held together during synapsis Figure 13.11-4 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Nonsister chromatids held together during synapsis Figure 13.11-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes Slide 33 3. Random fertilization Which sperm ends up fertilizing which egg is “random” • In humans: • There are 23 pairs of chromosomes, so there are 223 combinations during independent assortment • 223 = Slide 34 Variation in Humans • Independent Assortment • There are 23 pairs of chromosomes, so there are 223 possible combinations during independent assortment • 223 = 8,388,608 combinations of chromosomes from one meiotic event • Random fertilization • 8,388,608 x 8,388,608 = 70,368,744,177,664 • That’s 70.3 TRILLION possible combinations of sperm and eggs • Crossing over • Makes the variation essentially limitless Slide 35 Darwin vs. Mendel