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Meiosis

Meiosis. IB Biology. Match up the key words to their definitions and copy them into your notes!. Gene locus ( loci ). Homologous pair. Diploid. Gamete. Allele. Haploid. n=23. human sex cell. sperm. n=23. n=23. 2n=46. haploid (n). n=23. n=23. diploid (2n). n=23. meiosis I.

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Meiosis

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  1. Meiosis IB Biology

  2. Match up thekeywordstotheirdefinitions and copythemintoyour notes! Gene locus (loci) Homologouspair Diploid Gamete Allele Haploid

  3. n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23 n=23 diploid (2n) n=23 meiosis I meiosis II Spermatogenesis

  4. Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate.

  5. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

  6. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

  7. Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis occurs:homologous chromosomes come together to form a tetrad. • Tetrad is two chromosomes or four chromatids(sister and nonsister chromatids).

  8. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  9. Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus(position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

  10. eye color locus eye color locus hair color locus hair color locus Paternal Maternal Homologous Chromosomes

  11. Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over.

  12. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Over - variation variation

  13. Sex Chromosomes XX chromosome - female XY chromosome - male

  14. spindle fiber centrioles aster fibers Prophase I

  15. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

  16. OR metaphase plate metaphase plate Metaphase I

  17. Question: • In terms of Independent Assortment -how many different combinations of sperm could a humanmale produce?

  18. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations

  19. Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

  20. Anaphase I

  21. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

  22. Telophase I

  23. Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis

  24. Prophase II • same as prophase in mitosis

  25. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

  26. Anaphase II • same as anaphase in mitosis • sister chromatids separate

  27. Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

  28. Telophase II

  29. n=2 sex cell sperm n=2 n=2 2n=4 haploid (n) n=2 n=2 diploid (2n) n=2 meiosis I meiosis II Meiosis

  30. Variation • Important to population as the raw material for natural selection. • Question: What are the three sexual sources of genetic variation?

  31. Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

  32. Question: • A cell containing 20 chromosomes(diploid)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  33. Answer: • 10 chromosomes (haploid)

  34. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

  35. Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  36. Answer: • 10 chromosomes

  37. Now complete your Meiosis worksheet

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