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Ch 13 Sexual Life Cycles and Meiosis

Ch 13 Sexual Life Cycles and Meiosis. Reproduction and the Transmission of Traits. Living organisms Have ability to reproduce Heredity Transmission of traits from one generation to the next Variation Offspring often differ in appearance from parents and siblings

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Ch 13 Sexual Life Cycles and Meiosis

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  1. Ch 13 Sexual Life Cycles and Meiosis

  2. Reproduction and the Transmission of Traits • Living organisms Have ability to reproduce • Heredity Transmission of traits from one generation to the next • Variation • Offspring often differ in appearance from parents and siblings • Environmental and genetic influences • Genetics Scientific study of heredity and variation

  3. Progeny (or offspring) -Inherit chromosomes from parents -DNA in chromosomes encode the genes that specify characteristics For example eye color, blood type, predisposition to disease

  4. What are the mechanisms by which chromosomes are passed onto (eukaryotic) offspring? Asexual reproduction Sexual reproduction

  5. Asexual reproduction Bud forms by mitosis of parent cells Bud genetically identical to parent LE 13-2 Parent Bud 0.5 mm

  6. Sexual Reproduction Meiosis: formation of haploid gametes (eggs, sperm) in parents 2. Fertilization: fusion of gametes from opposite parents--> diploid zygote-->mitotic growth-->embryo-->adult

  7. HumanSexualReproduction Key Haploid gametes (n = 23) Haploid (n) Ovum (n) LE 13-5 Diploid (2n) Sperm cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Offspring genetically unique from parents Mitosis and development Multicellular diploid adults (2n = 46)

  8. How many chromosomes are inherited by human zygotes? 46 chromosomes or 2n= 46 n= the number of chromosomes in a set 23 from mother, maternal chromosomes 23 from father, paternal “

  9. Human Karyotype 5 µm Stained metaphase chromosomes from human somatic cell LE 13-3 Why stain metaphase chromosomes? Compact & easier to see com- pared to interphase

  10. Pair of homologous chromosomes 5 µm LE 13-3 Centromere Sister chromatids

  11. Can you tell the difference between a human male and female by the karyotype? Humans have 22 pairs of autosomes (non-sex chromosomes) 1 pair of sex chromosomes Male sex ch.: XY Female sex ch.: XX

  12. Human Karyotype 5 µm Male or female? LE 13-3

  13. If somatic cells have two sets of chromosomes (diploid), how do gametes end up with only one set (haploid)? Diploid precursors to the egg and sperm undergo meiosis in the testis and ovary.

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

  15. Key LE 13-4 Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

  16. MEIOSIS I: Separates homologous chromosomes METAPHASE I PROPHASE I ANAPHASE I LE 13-8ab Sister chromatids remain attached Centromere (with kinetochore) Chiasmata Sister chromatids Metaphase plate Spindle Microtubule attached to kinetochore Homologous chromosomes separate Tetrad Homologous chromosomes (red and blue) pair and exchange segments Homologous recombination (crossing-over) Tetrads line up Pairs of homologous chromosomes split up

  17. MEIOSIS II: Separates sister chromatids LE 13-8b TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow 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

  18. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I LE 13-9 Prophase Prophase I Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Metaphase Anaphase Sister chromatids separate during anaphase Anaphase I Homologues separate during anaphase I; sister chromatids remain together Telophase Cytokinesis 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

  19. Mitosis produces two genetically identical diploid cells Meiosis produces four genetically distinct haploid cells

  20. Mechanisms of Genetic Variation Among OffspringCaused by Sexual Reproduction • Crossing over (homologous recombination) (Prophase I) 2. Independent assortment of chromosomes (Metaphase I & II) 3. Random fertilization (post-meiosis)

  21. Nonsister chromatids Recombination Prophase I of meiosis Tetrad LE 13-11 Chiasmata, pl Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

  22. Independent Assortment Key Maternal set of chromosomes LE 13-10 Possibility 2 Possibility 1 Paternal set of chromosomes Metaphase I Two equally probable arrangements of chromosomes at Metaphase II Daughter cells Combination 3 Combination 2 Combination 4 Combination 1

  23. Random Fertilization • Any sperm can fuse with any ovum (unfertilized egg) • a (human) zygote has about 64 trillion diploid combinations • Each zygote has unique genetic identity

  24. I think they were talking about us. I hope they had it right! If not maybe we can answer questions.

  25. See meiosis animation What is missing?

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