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Meiosis

Meiosis. Chapter 13. Sexual Reproduction. Chromosomes are duplicated in germ cells Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Gametes meet at fertilization. Asexual Reproduction. Single parent produces offspring Mitosis

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Meiosis

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  1. Meiosis Chapter 13

  2. Sexual Reproduction • Chromosomes are duplicated in germ cells • Germ cells undergo meiosis and cytoplasmic division • Cellular descendents of germ cells become gametes • Gametes meet at fertilization

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

  4. Sexual Reproduction • Involves • Meiosis • Gamete production • Fertilization • Produces genetic variation among offspring

  5. Homologous Chromosomes Carry Different Alleles • Cell has two of each chromosome • One chromosome in each pair from mother, other from father • Paternal and maternal chromosomes carry different alleles

  6. Sexual Reproduction Shuffles Alleles • Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits • This variation in traits is the basis for evolutionary change

  7. Chromosome Number • Sum total of chromosomes in a cell • Germ cells are diploid (2n) • Gametes are haploid (n) • Meiosis halves chromosome number

  8. Meiosis: Two Divisions • Two consecutive nuclear divisions • Meiosis I • Meiosis II • DNA is not duplicated between divisions • Four haploid nuclei form

  9. Meiosis I Homologous chromosomes separate Each homologue in the cell pairs with its partner, then the partners separate In-text figurePage 165

  10. Meiosis II • The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) In-text figurePage 165

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

  12. Prophase I • Each duplicated chromosome pairs with homologue (sister chromatids with homologous chromosomes = tetrad) • Homologues swap segments (crossing over) • Each chromosome becomes attached to spindle Figure 10.4 Page 167

  13. Crossing Over • Each chromosome becomes zippered to its homologue • All four chromatids are closely aligned (tetrad) • Nonsister chromosomes exchange segments • The point of crossing over is called a chiasma. Figure 10.5Page 168

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

  15. Metaphase I • Spindles attach to kinetochores • Chromosomes (tetrads) are pushed and pulled into the middle of cell • The chromsomes line up on the plate two by two instead of single file like they do in mitosis. • The spindle is fully formed Figure 10.4 Page 167

  16. Random Alignment • 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

  17. Random Alignment • 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

  18. Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)

  19. 1 2 3 Possible ChromosomeCombinations or or or Figure 10.6Page 169

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

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

  22. Interkinesis- This is the time between meiosis I and meiosis II. The difference between interkinesis and interphase is that DNA does not replicate. Centrosomes with centrioles repli-cate. Cetrosomes are microtubules organizing centers

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

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

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

  26. 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

  27. Plant Life Cycle multicelled sporophyte mitosis zygote Diploid fertilization meiosis Haploid spores gametes multicelled gametophytes mitosis Figure 10.7Page 170

  28. Animal Life Cycle multicelled body mitosis zygote Diploid fertilization meiosis Haploid gametes Figure 10.7Page 170

  29. Cytokinesis in female gametes is different than males. Males result in four cells of equal size to become sperm cells, but in females, meiosis results in one large egg and three small polar bodies.

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

  31. A human egg with 3 polar bodies

  32. 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

  33. 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

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

  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. Prophase vs. Prophase I • Prophase (Mitosis) • Homologous pairs do not interact with each other • Prophase I (Meiosis) • Homologous pairs become zippered together and crossing over occurs

  37. Anaphase, Anaphase I, and Anaphase II • Anaphase I (Meiosis) • Homologous chromosomes separate from each other • Anaphase/Anaphase II (Mitosis/Meiosis) • Sister chromatids of a chromosome separate from each other

  38. 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

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