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Meiosis: The Process of Cell Division

Meiosis is a cell division process that produces gametes with half the number of chromosomes. This article explains the stages of meiosis, the differences between diploid and haploid cells, and the causes of genetic variation. It also covers the importance of gametes and the consequences of chromosomal abnormalities like trisomy and monosomy.

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Meiosis: The Process of Cell Division

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  1. Meiosis

  2. The Diploid Cell • Cell with ___ sets of chromosomes • Contains chromosomes for each _________ pair • __________ = Body Cells: Diploid • All human cells except ___________ are diploid • Where does the diploid number come from? • ____, where n is the number of chromosomes an individual has • What is n for humans? ______ 2n? ______

  3. The Haploid Cell • Cell containing ___ of each kind of chromosome • Examples: ___________________ • ___ set of chromosomes • _____ the number of chromosomes of diploid • ___ • 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 • __________ are produced in specialized body cells • Sperm and Egg Cells • __ divisions: _______________________ • Meiosis occurs in ____ cells, not _____ cells • __ daughter cells • Each cell has ____ 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 ___________ breaks off and attaches to _______________ on the _____________ chromosome • Permits __________________________ between maternal and paternal chromosomes • Occurs during _____________

  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 • + ____________________________ • - _____________________________

  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 • __________ divides • ____________________ closes off

  21. End Result: • ____ new cells that contain _____ of the original cells number of chromosomes • Creates _________ = _________

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

  23. Meiosis Animation • Meiosis Animation

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

  27. Nondisjunction • Failure of ___________________ to ___________ properly during meiosis. • __________________ of a homologous pair move to the ____________ of the cell. • 1 gamete has ___________________ • Or • 1 gamete is ______________________

  28. Nondisjunction Animation • Nondisjunction Animation Notes:

  29. Chromosome Pop Quiz! • What is a chromosome? • Where do we find them? • What do they look like? • How many pairs of chromosomes do humans have? • What determines if someone is male or female?

  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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