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1. Meiosis and chromosome number Steps in meiosis Source of genetic variation

1. Meiosis and chromosome number Steps in meiosis Source of genetic variation Independent alignment of homologues b. recombination. Gametes have a single set of chromosomes. Gametes are haploid, with only one set of chromosomes Somatic cells are diploid. Haploid gametes ( n = 23).

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1. Meiosis and chromosome number Steps in meiosis Source of genetic variation

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  1. 1. Meiosis and chromosome number • Steps in meiosis • Source of genetic variation • Independent alignment of homologues b. recombination

  2. Gametes have a single set of chromosomes • Gametes are haploid, with only one set of chromosomes • Somatic cells are diploid.

  3. Haploid gametes (n = 23) Egg cell • The human life cycle • Meiosis creates gametes • Mitosis of the zygote produces adult bodies Sperm cell MEIOSIS FERTILIZATION Diploidzygote (2n = 46) Multicellulardiploid adults (2n = 46) Mitosis anddevelopment Figure 8.13

  4. Meiosis reduces the chromosome number from diploid to haploid • Chromosomes are duplicated before meiosis • Then cell divides twice to form four daughter cells.

  5. MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes(withcentriolepairs) Microtubules attached tokinetochore Metaphaseplate Sister chromatidsremain attached Sites of crossing over Spindle Nuclearenvelope Sisterchromatids Tetrad Centromere(with kinetochore) Homologouschromosomes separate Chromatin Figure 8.14, part 1

  6. While paired, they cross over and exchange genetic information (DNA) • homologous pairs are then separated, and two daughter cells are produced • In meiosis I, homologous chromosomes are paired

  7. MEIOSIS II: Sister chromatids separate TELOPHASE IAND CYTOKINESIS TELOPHASE IIAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavagefurrow Sister chromatidsseparate Haploiddaughter cellsforming Figure 8.14, part 2

  8. sister chromatids of each chromosome separate • result is four haploid daughter cells • Meiosis II is essentially the same as mitosis

  9. MITOSIS MEIOSIS Diploid Diploid 1 gamete precursor somatic cell 2n 2n duplication 2 2n 2n 3 2n 2n 4 2n 2n division diploid haploid 5 2n 2n 1n 1n 6 division 7 1n 1n 1n 1n

  10. MITOSIS MEIOSIS PARENT CELL(before chromosome replication) Site ofcrossing over MEIOSIS I PROPHASE I Tetrad formedby synapsis of homologous chromosomes PROPHASE Chromosomereplication Chromosomereplication Duplicatedchromosome(two sister chromatids) 2n = 4 Chromosomes align at the metaphase plate Tetradsalign at themetaphase plate METAPHASE I METAPHASE ANAPHASE I TELOPHASE I ANAPHASETELOPHASE Sister chromatidsseparate duringanaphase Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together Haploidn = 2 Daughtercells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cellsof mitosis n n n n Daughter cells of meiosis II Figure 8.15

  11. Homologous chromosomes carry different versions of genes at corresponding loci • Each chromosome of a homologous pair comes from a different parent • Each chromosome thus differs at many points from the other member of the pair

  12. POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

  13. Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis

  14. tetrad

  15. Tetrad Chaisma Centromere Figure 8.18A

  16. MEIOSIS I PROPHASE I METAPHASE I ANAPHASE I END OF INTERPHASE

  17. MEIOSIS METAPHASE II TELOPHASE I PROPHASE II ANAPHASE II TELOPHASE II

  18. INDEPENDENT ASSORTMENT TELOPHASE II METAPHASE II METAPHASE I METAPHASE I

  19. SPERMATOGENESIS b OOGENESIS a spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte polar body meiosis ll spermatids polar bodies (will be degraded) egg

  20. Accidents during meiosis can alter chromosome number Nondisjunctionin meiosis I • Abnormal chromosome count is a result of nondisjunction • Either homologous pairs fail to separate during meiosis I Normalmeiosis II Gametes n + 1 n + 1 n – 1 n – 1 Number of chromosomes Figure 8.21A

  21. Or sister chromatids fail to separate during meiosis II Normalmeiosis I Nondisjunctionin meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

  22. Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Eggcell n + 1 Zygote2n + 1 Spermcell n (normal) Figure 8.21C

  23. Connection: An extra copy of chromosome 21 causes Down syndrome • This karyotype shows three number 21 chromosomes • An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B

  24. The chance of having a Down syndrome child goes up with maternal age Figure 8.20C

  25. Connection: Abnormal numbers of sex chromosomes do not usually affect survival • Nondisjunction can also produce gametes with extra or missing sex chromosomes • Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

  26. Table 8.22

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