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

1. Meiosis and chromosome number Steps in meiosis Source of genetic variation Independent alignment of homologues b. Recombination. Somatic cells are diploid. Gametes are haploid, with only one set of chromosomes. Meiosis reduces the number of genomes from diploid to haploid.

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

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

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

  3. Meiosis reduces the number of genomes from diploid to haploid Haploid gametes (n = 23) Egg cell • 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. Steps in meiosis I 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

  5. 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

  6. 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

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

  8. 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

  9. Homologous chromosomes carry different versions of genes (alleles) 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

  10. Independent alignment of homologous chromosomes 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

  11. Crossing over further increases genetic variability • the exchange of corresponding segments between two homologous chromosomes

  12. Tetrad Chaisma Centromere Figure 8.18A

  13. MEIOSIS I PROPHASE I METAPHASE I ANAPHASE I END OF INTERPHASE Genetic recombination results from crossing over during prophase I of meiosis

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

  15. INDEPENDENT ASSORTMENT TELOPHASE II METAPHASE II METAPHASE I METAPHASE I

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

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

  22. 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

  23. Table 8.22

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