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  1. 1. 11-4 Meiosis P275
  2. 2. Homologous chromosomes• Pair of chromosomes that one comes from the male parent and the corresponding Homologous one from the female parent chromosomes Homologous ≠ identical Alleles same genes in same loci same length same shape may not the same alleles for one gene
  3. 3. Diploid & Haploid Cells• two copies of each • one copy of each chromosome chromosome• homologous pairs • no homologous pairs• body cells • gametes (sex cells) Sperm Egg
  4. 4. ? meiosis Sperm EggEgg Zygote fertilisation Sperm
  5. 5. The purpose of meiosisis to produce gametes.A (haploid) gamete(sex cell, sperm or eggcell) has half thenumber ofchromosomes comparedto a (diploid) somaticcell (body cell). A maleand a female gametemay then fuse to form azygote, which will havethe same number ofchromosomes as asomatic cell.
  6. 6. Human chromosomes– Diploid cells: 23 pairs of homologous chromosomes– 1st-22nd pairs: autosomes– 23rd pair: sex chromosome
  7. 7. Human chromosomes
  8. 8. 1-22 pairs : autosomes, 23 pair : sex chormosome
  9. 9. Karyotype (the type of nucleus)• The number and appearance of the chromosomes in an organism
  10. 10. Karyotyping• Arrange the chromosomes in pairs according to their size and structure • Collect cells • Culture cells • Stop cell division at metaphase • Staining chromosome • Pair (Length, Shape, Position of centromeres, Bands) • Diagnosis (number, gender, abnormality)
  11. 11. How many chromosoems 8 How many pairs of homologous chromosomes ? 4 How many pairs of autosomes ? 3 How many pairs ofFruit fly’s somatic cells sex chromosomes ? 1 : x,y
  12. 12. A Bgerm cell C D What is the correct gamete produced by the germ cell
  13. 13. Meiosis Overview• reduction division• number of chromosomes per cell halved• separation of homologous chromosomes• 1 diploid  4 genetically different haploid• DNA copied once, cell divide twice
  14. 14. Sperms are produced by … meiosisDiploid parent cell The chromosomes in the nucleus are copied Four haploid daughter cells
  15. 15. Diploid Eggs are also produced byparent cell meiosis Haploid
  16. 16. Phases of meiosis MEIOSIS I MEIOSIS II PROPHASE I PROPHASE II interphase no interphase(DNA replication METAPHASE I (no DNA replication METAPHASE IIbefore meiosis I) ANAPHASE I before meiosis II) ANAPHASE II TELOPHASE I TELOPHASE II
  17. 17. Meiosis I (reduction division)Interphase I Prophase I Metaphase I Anaphase I Telophase ICells undergo a round Each chromosome pairs Spindle fibers attach to The fibers pull theof DNA replication, with its corresponding the chromosomes. homologous chromosomesforming duplicate homologous chromosome toward the opposite endsChromosomes. to form a tetrad. of the cell.
  18. 18. Prophase 1• Supercoil of chromosomes• Spindle form• Synapsis (homologous pair to form bivalents)• Crossing over• Nuclear envelope breaks down
  19. 19. Synapsis & Crossing over • The key to the process of meiosis is the Synapsis in Prophase I and the splitting of the homologues in Anaphase I. • One event that can take place during meiosis is crossing over. • Crossing over is the exchange of genetic material between two homologous chromosomes which have paired up during Prophase I. • The result of crossing over is genetic recombination.
  20. 20. Metaphase 11. Spindle network complete2. Bivalents line up on equator3. Centromere attached to microtubules
  21. 21. Anaphase 11. Centromeres do not split2. Microtubules contract to separate homologous pairs
  22. 22. Telophase 11. Chromosomes uncoil2. Spindle breaks down3. Nuclear envelope reform
  23. 23. Meiosis II (similar to mitosis)There is no DNAreplication Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two The chromosomes line up The sister chromatids Meiosis II results in four haploid (N) daughter cells, in a similar way to the separate and move toward haploid (N) daughter cells. each with half the number metaphase stage of opposite ends of the cell. of chromosomes as the mitosis. original.
  24. 24. Prophase 21. Supercoil of chromosomes2. Spindle form3. Nuclear envelope breaks down
  25. 25. Metaphase 21. Chromosomes move to equator2. Spindle network complete3. Centromere attach to spindle
  26. 26. Anaphase 21. Centromere split2. Microtubules contract and sister chromatids separate
  27. 27. Telophase 21. Chromosomes uncoil2. Spindle breaks down3. Nuclear envelope reformsCytokinesis 21. 4 haploid cells2. 1 or many undergo differentiation into gametes
  28. 28. Which steps in meiosis are important in producing genetic variety?
  29. 29. 1. Crossing over in prophase 1• Homologous chromosomes exchange nonsister chromatids chiasmata
  30. 30. 2. Independent assortment in metaphase 1• Bivalents line up on equator in a random way
  31. 31. Possibilities of Crossing over
  32. 32. e.g. 46 chromosomes, 23 pairs2n = 46; n = 232n = 223 = ~ 8 million possible combinations!
  33. 33. 3. Random fertilization
  34. 34. Gamete formation in animals• Male gametes (sperms) are formed through spermatogenesis• Female gametes (egg) are formed through oogenesis
  35. 35. a) Spermatogenesisspermato- primary secondary spermatidsgonium spermatocyte spermatocytes sperm (n) (n)(2n ) (2n) (n)Growth Meiosis I Meiosis II
  36. 36. b) Oogenesis polar body polar (n) bodies (n)oogonium primary oocyte (2n) (2n) secondary oocyte (n) ovum (n) Growth Meiosis I Meiosis II
  37. 37. Comparing mitosis and meiosis• Mitosis: 2 genetically identical diploid cells• Meiosis: 4 genetically different haploid cells