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Meiosis

Meiosis. Chapter 11. Asexual Reproduction . Single parent produces offspring All offspring are genetically identical to one another and to parent. Sexual Reproduction. Two Parents produce offspring Involves: Meiosis Gamete production Fertilization

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Meiosis

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  1. Meiosis Chapter 11

  2. Asexual Reproduction • Single parent produces offspring • All offspring are genetically identical to one another and to parent

  3. Sexual Reproduction • Two Parents produce offspring • Involves: • Meiosis • Gamete production • Fertilization • Produces genetic variation among offspring

  4. Chromosome Number • Each species has a unique number of chromosomes • Humans have 46 chromosomes • This number actually represents 2 sets of chromosomes • One set from your mother & one from your father • Each of these sets is called Haploid or n. • Humans have 23 pairs of homologous chromosomes, this is described as being Diploid or 2n

  5. Homologous Chromosomes • Cell has two of each chromosome • One chromosome in each pair from mother, other from father • Homologous chromosome have similar structure • They carry the same traits (e.g. hair color), but may carry different alleles (different versions of the same trait; e.g. blond vs. brown hair)

  6. Why do organisms do Meiosis? • To preserve the chromosome number of cells in organism undergoing sexual reproduction • Meiosis results in haploid cells called gametes

  7. Where do organisms perform Meiosis? In animals, Meiosis only occurs in organs called gonads (sex organs) • Testes • Male gonads; make sperm (male gametes) • Ovaries • Female gonads; make eggs (ova, female gametes)

  8. Gamete Formation ovaries anther ovary testes Figure 10.2aPage 164

  9. Sexual Reproduction • Chromosomes are duplicated in germ cells (diploid) • Germ cells undergo meiosis and cytoplasmic division • Cellular descendents of germ cells become gametes (haploid) • Gametes meet at fertilization to form zygotes (diploid)

  10. Sexual Reproduction

  11. Meiosis: Two Divisions • Two consecutive nuclear divisions • Meiosis I • Meiosis II • DNA is not duplicated between divisions • Results in a reduction of the chromosome number from diploid to haploid (cuts the chromosome number in half

  12. Meiosis I • Homologous chromosomes bind to each other and are later separated on from another Each homologue in the cell pairs with its partner, then the partners separate In-text figurePage 165

  13. Meiosis II • The two sister chromatids of each duplicated chromosome are separated from each other, resulting in four haploid cells two chromosomes (unduplicated) one chromosome (duplicated) In-text figurePage 165

  14. Meiosis – Overview

  15. Prophase I Metaphase I Anaphase I Telophase I Meiosis I - Stages Figure 10.4 Page 167

  16. Prophase I • Tetrad formation: Each duplicated chromosome pairs with homologue • Crossing Over: Homologues swap segments • Each chromosome becomes attached to spindle Figure 10.4 Page 167

  17. Metaphase I • Chromosomes are pushed and pulled into the middle of cell • The spindle is fully formed Figure 10.4 Page 167

  18. Anaphase I • Homologous chromosomes segregate • The sister chromatids remain attached Figure 10.4 Page 167

  19. Telophase I • The chromosomes arrive at opposite poles • Usually followed by cytoplasmic division Figure 10.4 Page 167

  20. Prophase II • Microtubules attach to the kinetochores of the duplicated chromosomes Figure 10.4 Page 167

  21. Metaphase II • Duplicated chromosomes line up at the spindle equator, midway between the poles Figure 10.4 Page 167

  22. Anaphase II • Sister chromatids separate to become independent chromosomes Figure 10.4 Page 167

  23. Telophase II • The chromosomes arrive at opposite ends of the cell • A nuclear envelope forms around each set of chromosomes • Four haploid cells Figure 10.4 Page 167

  24. three polar bodies (haploid) Oogenesis first polar body (haploid) oogonium (diploid) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth Figure 10.8Page 171

  25. Spermatogenesis primary spermatocyte (diploid) sperm (mature, haploid male gametes) spermato-gonium (diploid ) secondary spermatocytes (haploid) spermatids (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth cell differentiation, sperm formation Figure 10.9Page 171

  26. Crossing Over: Homologous Recombination • Unique to meiosis does NOT happen during mitosis • Swapping of DNA between two homologous chromosomes • A piece of DNA from a chromosome from dad swaps places with a piece of DNA from the chromosome from mom • This can occur when chromosomes are aligned during prophase I, takes place between nonsister chromatids of homologous chromosomes • This leads to genetic recombination which in turn leads to variation in the traits of offspring

  27. Effect of Crossing Over • After crossing over, each chromosome contains both maternal and paternal segments • Creates new allele combinations in offspring

  28. Random Segregation • During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes • Initial contacts between microtubules and chromosomes are random

  29. Random Segregation • Either the maternal or paternal member of a homologous pair can end up at either pole • The chromosomes in a gamete are a mix of chromosomes from the two parents

  30. 1 2 3 Possible ChromosomeCombinations or or or Figure 10.6Page 169

  31. Fertilization • Male and female gametes unite and nuclei fuse • Fusion of two haploid nuclei produces diploid nucleus in the zygote • Which two gametes unite is random • Adds to variation among offspring

  32. Factors Contributing to Variation among Offspring • Crossing over during prophase I • Random alignment of chromosomes at metaphase I • Random combination of gametes at fertilization

  33. Variation: The numbers Germ Cell (♂) Germ Cell (♀) Sperm X Egg (223combinations) (223combinations) Zygote ~70 trillion combinations Meiosis Random Assortment Random Fertilization

  34. Variation • Independent assortment gives rise to tremendous variation • 2n = the number of possible combinations for each gamete (n = Chromosome number) • For humans: 223 = # combinations for each egg or sperm = 8.4 x 106 • Random fertilization: 8.4 x 106 possible eggs + 8.4 x 106 possible sperm = 70 trillion possible combinations!!!

  35. Mitosis Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring Mitosis & Meiosis Compared

  36. Results of Mitosis and Meiosis • Mitosis • Two diploid cells produced • Each identical to parent • Meiosis • Four haploid cells produced • Differ from parent and one another

  37. Then what: Early Development Cleavage (mitosis) (Zygote)

  38. Early Development - Starfish Zygote Cleavage Morula Blastula Gastrulation

  39. Nondisjunction: When Meiosis Goes Wrong n + 1 n + 1 n - 1 chromosome alignments at metaphase I n - 1 nondisjunction at anaphase I alignments at metaphase II anaphase II Figure 12.17Page 208

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