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Understanding Meiosis: The Process of Cell Division

Learn about meiosis, the process of cell division that creates haploid gametes with half the number of chromosomes, and how it is different from mitosis.

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Understanding Meiosis: The Process of Cell Division

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  1. Meiosis

  2. The Diploid Cell • Cell with 2 sets of chromosomes • Contains chromosomes for each homologous pair • Somatic Cells = Body Cells: Diploid • All human cells except sex cells are diploid • Fertilization restores the diploid number • 2n, where n is the number of chromosomes an individual has • Humans: n = 23, 2n = 46

  3. The Haploid Cell • Cell containing one of each kind of chromosome • Ex. sperm and egg cells (gametes) • 1 set of chromosomes • Half the number of chromosomes of diploid • n • Sperm + Egg = Zygote

  4. Haploid = one of each kind Diploid = two of each kind • 2n = diploid n = haploid + or

  5. Mitosis vs. Meiosis • Mitosis: t = two: Diploid 2n=46 • Meiosis: o = one: Haploid • n = 23

  6. Meiosis • Gametes are produced in specialized body cells • Sperm and Egg Cells • 2 divisions: Meiosis I and Meiosis II • Meiosis occurs in sex cells, not body cells • 4 daughter cells • Each cell has half the chromosomes of the parent

  7. Meiosis I • Interphase occurs: the cell grows and DNA replicates • Meiosis I begins • Original cell produces two new cells • Like Mitosis

  8. Prophase I • DNA Coils tightly into chromosomes • Spindle fibers appear • Each chromosome lines up next to the homologue • Synapis occurs: pairing of homologous chromosomes • Tetrad: Each pair of homologous chromosomes

  9. Crossing Over • Crossing Over: why we do not look exactly like our parents. • Portions of the chromatid breaks off and attaches to adjacent chromatids on the homologous chromosome • Permits the exchange of genetic material between maternal and paternal chromosomes • Occurs during Prophase

  10. Genetic Recombination • Crossing over produces a new mixture of genetic material

  11. Causes of Variation • Chromosomes are assorted randomly • Crossing over may occur • Cells do not have identical genetic info as each other or the parent • + more chance of survival and evolution • - mistakes more likely

  12. Metaphase I • Tetrads line up randomly along the mid-line • Spindle fibers attach to centromeres

  13. Anaphase I • Homologous chromosomes move to the opposite poles • Random separation or Independent Assortment results in separation of maternal and paternal chromosomes.

  14. Telophase I • Chromosomes reach opposite ends of cell • Cytokinesis begins • Cell is now haploid

  15. Meiosis II • Occurs in each cell formed in Meiosis I • Interphase does not occur again

  16. Prophase II • Spindle fibers form and move the chromosomes to the mid-line of the dividing cell

  17. Metaphase II • Chromosomes move to the mid-line of the dividing cell facing opposite poles of the dividing cell

  18. Anaphase II • Chromatids separate and move to opposite poles of the cell

  19. Telophase II • Nuclear membrane forms around the chromosomes in each of 4 new cells

  20. Cytokinesis II • Cytoplasm divides • Cell membrane closes off

  21. End Result: • Four new cells that contain half of the original cells number of chromosomes • Creates sex cells = gametes

  22. What exactly is a gamete? • Formed by meiosis • Haploid reproductive cells • In humans, meiosis occurs in the testes and ovaries

  23. Meiosis Animation • Meiosis Animation

  24. Why do we need haploid gametes? • EGG Female gamete Male gamete 23 23 SPERM

  25. Fertilization restores the diploid number 1n meiosis 1n fertilization 2n 2n Mitosis and cell growth 2n

  26. Mitosis vs. Meiosis • Mitosis vs. Meiosis Animation

  27. Nondisjunction • Failure of homologous chromosomes to separate properly during meiosis. • Both chromosomes of a homologous pair move to the same pole of the cell. • 1 gamete has an extra chromosome • Or • 1 gamete is missing a chromosome

  28. Nondisjunction Animation • Nondisjunction Animation

  29. Chromosome Pop Quiz! • Chromosome: • Rod-shaped coiled and compact DNA • “Colored body” • Found in the nucleus • Looks like X • Humans have 23 pairs of chromosomes • Female = XX, Male = XY (23rd pair)

  30. Trisomy: Extra Chromosomes • Trisomy 13: Patau’s Syndrome • Trisomy 21: Down’s Syndrome • XXX: Trisomy X • XYY: Jacob’s Syndrome • XXY: Klinefelter’s Syndrome

  31. Monosomy: Missing Chromosomes • Most do not survive • Turner XO : develop as girls, but infertile • OY: lethal

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