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Genetics III: Meiosis and Sexual Reproduction

Genetics III: Meiosis and Sexual Reproduction. The Reproduction of Organisms. Asexual reproduction involves Mitosis a) single-celled organisms reproduce by dividing 1 cell into 2 b) Some multicellular organisms can divide into pieces that then grow into new individuals. (b) Sea star.

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Genetics III: Meiosis and Sexual Reproduction

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  1. Genetics III: Meiosis and Sexual Reproduction

  2. The Reproduction of Organisms Asexual reproduction involves Mitosis a) single-celled organisms reproduce by dividing 1 cell into 2 b) Some multicellular organisms can divide into pieces that then grow into new individuals (b) Sea star (a) Amoeba Figure 8.2a Figure 8.2b

  3. Mitosis does not work for organisms who use sexual reproduction • Sexual reproduction requires the union of gametes • (sperm and egg) If human sperm and egg cells were produced by mitosis they would have an identical full copy of all chromosomes… 46 92 46 + = Gamete formation must involve a reduction in chromosome number

  4. Sexual reproduction involves Meiosis and Fertilization • Production of gametes (egg and sperm) is called meiosis • The fusion of the sperm and egg is called fertilization • Fertilization creates a zygote, or fertilized egg • Somatic cells (non-sex cells) are produced by mitosis • Gametes (sex cells) are produced by meiosis

  5. Life Cycle of a Sexual Organism Haploid gametes (n = 23) Egg cell Sperm cell Meiosis Fertilization Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

  6. I. How are the gametes produced? Meiosis- division of the nucleus in sex cells • produces sex cells (gametes) • halves the number of • chromosomes (haploid) • Two nuclear divisions • Meiosis I- separation of homologous • chromosomes • Meiosis II- separation of chromatids • Four new cells are produced and each cell is • genetically unique

  7. Homologous chromosomes are matching pairs of chromosomes Pair of homologous chromosomes Centromere Sister chromatids • These come together in Meiosis 1: synapsis Figure 8.12

  8. Crossing Over Tetrad • In crossing over Prophase I of meiosis • Homologous chromosomes exchange genetic information • Genetic recombination occurs Chiasma, site of crossing over Metaphase I Spindle microtubules Metaphase II Gametes Figure 8.18 Recombinant chromosomes

  9. Meiosis consists of 2 divisions and results in four haploid daughter cells Parent Cell (2n) Four Haploid (n) cells Fertilization will restore the full diploid number.

  10. Stages of Meiosis: Meiosis I Telophase I and Cytokinesis I Interphase (4 chromosomes) Prophase I Metaphase I Anaphase I* 2 cells each with 4 chromatids * sister chromatids remain attached

  11. Interphase II: NO Replication! Telophase II and Cytokinesis II Meiosis II Anaphase II* Prophase II Metaphase II 4 cells each with 2 chromosomes * chromatids split at centromere

  12. II. Ways in which Meiosis contributes to genetic variation A. Metaphase I Alignments - chromosomes may line up at the metaphase plate in any number of ways; therefore, we may see different combinations of chromosomes in the gametes.

  13. B. Crossing Over - “Recombinant” chromosomes - this creates new combinations of genes that did not exist before C. Random Fertilization - which gametes are actually involved in fertilization is left up to chance

  14. Comparing Mitosis and Meiosis Mitosis Meiosis Type of cells produced # chromosomes in daughter cells as compared to # in parent cell # nuclear divisions Number of daughter cells produced somatic gametes Same 2n  2n Half 2n  n 1 2 2 4

  15. III. Problems with cell division can lead to disorders A. Incorrect Numbers of Chromosomes Non-disjunction occurs when chromosomes do not separate properly during anaphase Gametes with an incorrect number of chromosomes are produced

  16. Down Syndrome • Any missing or extra autosome is usually fatal, but chromosome #21 is very short • an extra chromosome 21 is also called trisomy 21 • These individuals have Down Syndrome Figure 8.19

  17. Abnormal numbers of sex chromosomes • A missing X chromosome is fatal (YO) • Any individual with a Y chromosome is a male • Individuals can survive with too many or too few sex chromosomes but have abnormalities: Table 8.1

  18. Study Objectives 1. Explain why mitosis does not work for gamete formation. 2. Explain what happens in the various stages of meiosis I and meiosis II. 3. Explain how sexual reproduction leads to genetic variation (metaphase alignments, random fertilization, crossing over). 4. Contrast mitosis and meiosis in terms of a) the types of cells produced by each process, b) the number of chromosomes in the cells produced as compared to the original cell, c) the number of nuclear divisions, and d) the number of daughter cells produced. 5. Define non-disjunction. Give examples of some disorders that may result from non-disjunction.

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