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Meiosis

Meiosis. Production of Sex Cells. Meiosis. A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Diploid – 2 sets of chromosomes Haploid – 1 set of chromosomes

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Meiosis

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  1. Meiosis Production of Sex Cells

  2. Meiosis • A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. • Diploid– 2 sets of chromosomes • Haploid – 1 set of chromosomes • Homologous –corresponding chromosomes from each parent

  3. How to draw a chromosome

  4. Homologous Chromosomes One came from the other parent One came from one parent Both contain they same types of genes (i.e. gene for eye color)

  5. karyotype

  6. Diploid Cells (2n) 22 pairs of homologous chromosomes and 1 pair of sex chromosomes • All the cells that make up the body EXCEPT sex cells • Humans have 46 chromosomes in all body (somatic) cells

  7. Haploid Cells (n) • Sex cells • In animals, eggs and sperm • Half the # of chromosomes as the somatic cells

  8. Zygote = fertilized egg cell sperm (haploid) + egg (haploid) = zygote (diploid) 23 + 23 = 46 Once the zygote is produced, it begins to divide using mitosis

  9. Meiosis- how sex cells are made

  10. Meiosis Stages • Meiosis involves 2 distinct stages • Meiosis I • Meiosis II

  11. Prophase I • Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. • Crossing over may occur here

  12. Prophase I: Crossing over • when chromosomes overlap and exchange sections of their genetic information

  13. Prophase I Chromosomes become visible crossing-over occurs meiotic spindle forms

  14. Metaphase I • Spindle fibers attach to the chromosomes

  15. Anaphase I • The fibers pull the homologous chromosomes toward opposite ends of the cell.

  16. Telophase I & Cytokinesis • Nuclear membranes form. • The cell separates into 2 cells.

  17. Summary of Meiosis I • Meiosis I animation • Reduction of chromosomes from 1 diploid (2n) cell to 2 haploid (n) cells

  18. Prophase II • Now two cells • Each cell now has half the number of chromosomes as the original cell. • Spindle forms again

  19. Metaphase II • The chromosomes line up similar to metaphase in mitosis.

  20. Anaphase II • Sister chromatids separate and move to opposite ends of the cell.

  21. Telophase II • Meiosis II results in 4 haploid cells. • Summary of Meiosis II

  22. Gamete Formation • In males, meiosis results in 4 sperm cells • In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction.

  23. Gametogenesis • Spermatogenesis • 4 tiny haploid sperm • Oogenesis • 1 large haploid egg, 3 small polar bodies

  24. Meiosis Summary • Two cell divisions • Meiosis I (PMAT I) • Prior to meiosis I each chromosome is replicated • Homologous chromosomes separate • Results in 2 haploid cells • Meiosis II (PMAT II) • Sister chromatids separate • Results in 4 haploid cells

  25. Creating Variation • Meiosis creates genetic variation in two ways: • Random shuffling of chromosomes in Metaphase I • More than 8 million possible gametes • Crossing over in Prophase I • Homologous chromosomes swap pieces of DNA when tightly paired as tetrad

  26. Mitosis vs Meiosis

  27. Polyploidy • Organisms with more than usual number of chromosomes • Trisomy 21 (3 copies of chromosome 21) • Called Down’s syndrome

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