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DRILL #14

DRILL #14. Quote: You will never leave where you are , until you decide where you'd rather be. ~Dexter Yager Agenda : Meiosis Notes Warm-up: 1. Describe interphase .

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DRILL #14

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  1. DRILL #14 Quote: You will never leave where you are, until you decide where you'd rather be. ~Dexter Yager Agenda:Meiosis Notes Warm-up: 1. Describe interphase. 2. After a bacterial cell has undergone binary fission, how many chromosome copies will each daughter cell contain?

  2. Meiosis Notes CH 11 Go to Section:

  3. Meiosis • The form of cell divisionby which gametes, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II).

  4. Meiosis • Sex cells divide to produce gametes(sperm or egg). • Gametes have half the # of chromosomes. • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences.

  5. 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

  6. 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.

  7. 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

  8. 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).

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

  10. 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. Having two copies of every gene can be an advantage. For example, if the one gene from Dad was damaged, Mom’s good gene could take over. • 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

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

  12. 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.

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

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

  15. spindle fiber centrioles aster fibers Prophase I

  16. 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

  17. OR metaphase plate metaphase plate Metaphase I

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

  19. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223= 8,388,608 million combinations • Any one of these assortments can combine with any one of the 8,388,608 combinations of his partner!

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

  21. Anaphase I

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

  23. Telophase I

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

  25. Prophase II • same as prophase in mitosis

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

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

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

  29. Telophase II

  30. 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

  31. In males, four spermare produced and flagella soon develop. In females, one large eggsurvives and three smaller eggs die.

  32. 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

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

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

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

  36. Answer: • 10 chromosomes (haploid)

  37. Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

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

  39. Answer: • 10 chromosomes

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