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Meiosis

Meiosis. A presentation by: Ms. Edmondson. Part 1: Introduction. Mitosis Reminder. Mitosis is the process of cell division. One cell divides to produce two identical cells. So what is Meiosis?. Meiosis is a special kind of cell division.

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Meiosis

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  1. Meiosis A presentation by: Ms. Edmondson

  2. Part 1: Introduction

  3. Mitosis Reminder • Mitosis is the process of cell division. • One cell divides to produce two identical cells.

  4. So what is Meiosis? • Meiosis is a special kind of cell division. • It occurs when one cell divides twice to produce four different cells

  5. Mitosis Meiosis • Cell divides once • Two genetically identical cells are made. • Daughter cells have same genetic information. • Occurs in body cells • Cell divides twice • Four genetically different cells are made. • Daughter cells have half of the genetic information. • Occurs in sex cells

  6. Vocabulary Gene- Basic unit of heredity Diploid- A cell with two copies of each chromosome. Haploid- A cell with one copy of each chromosome. Gamete- A sperm or egg cell Zygote- Fertilized egg

  7. Why don’t all cells use mitosis? Let’s look at an example!

  8. n=23 Zygote Egg (Female gamete) Sperm (Male gamete) n=23 Fertilization • The fusion of a sperm and eggto form a zygote. • A zygote is a fertilized egg n=46

  9. So…why don’t all cells use mitosis? • During sexual reproduction: • Organisms get a set of chromosomes from their mom and a set of chromosomes from their dad. • What would happen if a human baby’s cells got 46 chromosomes from their mom and 46 chromosomes from their dad? • They would end up with 92 chromosomes! • Meiosis allows gametes to have only half of the genetic information available (# of chromosomes) • Based on this information, how many chromosomes should be present in gametes? • 23 chromosomes

  10. Mitosis vs. Meiosis • Occurs in body cells • Results in 2 daughter cells each with 46 chromosomes (in humans) • These chromosomes are called diploid • Occurs in sex cells • Results in 4 daughter cells (gametes) each with 23 chromosomes (in humans) • These chromosomes are called haploid

  11. Part 2: Haploid vs. Diploid

  12. Diploid • Diploid cells have two copies of each chromosome, one from mom and one from dad. • We write diploid as 2n.

  13. Egg (Female gamete) 1n Zygote 2n Sperm (Male gamete) 1n Haploid • Haploid cells have one copy of each chromosome. • We write haploid as 1n

  14. Review Question: • Remember, just because human’s need 23 pairs of chromosomes, all organisms do not. • A cell containing 20 chromosomes(diploid)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  15. Answer: • 10 chromosomes!

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

  17. Answer: • 20 chromosomes!

  18. What does meiosis look like? • Diploid: • Having two copies of each chromosome • 2n= diploid • Haploid: • Having one copy of each chromosome • 1n= haploid 2n Meiosis I Meiosis 2 1n

  19. Practice! Let’s do some haploid diploid practice!

  20. Part 3: Homologous Chromosomes

  21. Homologous Shoes? • Let’s start by having everyone take off their shoes! • Now…hold them in the air. • Other than the stench, what do you notice?

  22. Homologous Shoes? Homologous Chromosomes Left (maternal) Left (maternal) Left (maternal) Right (paternal) Right (paternal) Right (paternal) Homologous Chromosomes Homologous Chromosomes

  23. Homologous Chromosomes • Homologous Chromosomes - Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Humans have 23 pairs of chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes • XX- female • XY- male Homologous Shoes Homologous Chromosomes maternal paternal left right

  24. Paternal Maternal (Dad)(Mom) Homologous Chromosomes eye color gene eye color gene hair color gene hair color gene

  25. Homologous chromosomes Tetrad Tetrads

  26. Part 4:Phases of Meiosis

  27. Interphase I • Similar to what we learned when we studied mitosis • G1= Growth • S-phase: DNA is replicated • G2: Growth and preparation for meiosis • Each replicated chromosome consist of two identical sister chromatids attached at their centromeres

  28. Stages of Meiosis • Meiosis involves 2 divisions • Meiosis I and Meiosis II • Meiosis I- Reduces the number of chromosomes in each daughter cell in half • Meiosis II- Very similar to mitosis.

  29. Meiosis Stages • Meiosis goes through mitosis twice. • There are only a few differences and I will point those out as we go along.

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

  31. Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. • Four phases: 1.) Prophase I 2.) Metaphase I 3.) Anaphase I 4.) Telophase I

  32. Prophase I • Longest and most complex phase of meiosis • (90%) of the cell’s time is spent in this phase! • Chromosomes condense. • There are two major differences between prophase of mitosis and prophase I of meiosis occur in this phase: 1.) Synapsis 2.) Crossing Over

  33. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis • Synapsis: Homologous chromosomes come together to form a tetrad. • Tetrad= 2 replicated chromosomes= four chromatids= 2 sister & 2 nonsisterchromatids

  34. Prophase I – Crossing Over Segments of non-sister chromatids break and reattach to the other chromatid. • Crossing over (variation) may occur between non-sister chromatids at the chiasmata. • Chiasmata (chiasma) are the sites of crossing over.

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

  36. spindle fiber centrioles aster fibers Prophase I:the end result

  37. Prophase: Mitosis vs. Meiosis • Nuclear membrane degenerates • Homologous chromosomes pair off into tetrads • Crossing over occurs between like parts of non-sister chromatids • Nuclear membrane degenerates

  38. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • Independent Assortment Occurs: 1. Placement of homologous pair is random.

  39. OR metaphase plate metaphase plate Metaphase I

  40. Metaphase: Mitosis vs. Meiosis • Single chromosomes line up along the metaphase plate • Spindle fibers emerge from the both centrioles and attach to both sides of the centromere • Tetrads line up along the metaphase plate • Spindle fibers emerge from both centrioles and attach to one side of the centromere

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

  42. Anaphase I

  43. Anaphase: Mitosis vs. Meiosis • Sister chromatids are pulled to opposite poles of the cell • Sister chromatids move together to the same pole of the cell • Homologous chromosomes are pulled to opposite poles of the cell

  44. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis may or may not occur and two haploid daughter cells are formed.

  45. Telophase I

  46. Telophase: Mitosis vs. Meiosis • Each daughter cell is diploid • This is the end of mitosis • Each daughter cell is haploid

  47. Meiosis II • No interphase II (No more DNA replication) Has four stages: 1.) Prophase II 2.) Metaphase II 3.) Anaphase II 4.) Telophase II Remember: Meiosis II is similar to mitosis

  48. Prophase II • Same as prophase in mitosis • Nuclear membrane degenerates • New spindle fibers form

  49. metaphase plate metaphase plate Metaphase II • Same as metaphase in mitosis • Chromosomes line up along metaphase plate • Spindle fibers bind to each side of the centromere

  50. Anaphase II • Same as anaphase in mitosis • Spindle fibers shorten and pull sister chromatids apart

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