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What is Meiosis?

What is Meiosis? . MEIOSIS is the process by which reproductive cells, also called gametes , are formed. Very similar to mitosis, but cells divide 2 times , so gametes have half the number of chromosomes of body ( somatic ) cells.

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What is Meiosis?

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  1. What is Meiosis? MEIOSIS is the process by which reproductive cells, also called gametes, are formed. Very similar to mitosis, but cells divide 2 times, so gametes have half the number of chromosomes of body (somatic) cells.

  2. Meiosis in animals produces sperm & eggs. Meiosis in plants and fungi produces spores.

  3. Haploid number (n): half the number of chromosomes (n=23 in humans) –the sperm and the egg. Zygote= fertilized egg, which is diploid (has a full set of chromosomes 46 in humans

  4. How is meiosis beneficial to the survival of organisms? • Meiosis increases genetic variability because offspring (baby) is genetically different from both parents.

  5. In meiosis, the number of chromosomes per cell is cut in half through the separation of homologous chromosomes. Meiosis has 2 phases: meiosis I and meiosis II

  6. Interphase I • DNA replicates, forming duplicate chromosomes.

  7. Meiosis I

  8. MEIOSIS I Prophase I • Nuclear membrane disappears • Spindle forms • Each chromosome (which consists of 2 sister chromatids) pairs with its homologous chromosome to form a tetrads.

  9. MEIOSIS I Prophase I • A tetrad forms when homologous sister chromatids pair up Sister chromatids from biological mother Sister chromatids from biological father -When did the sister chromatid appear? -What is a homologous chromosome? -Where did each chromosome in the homologous pair originate? -What is a tetrad?

  10. Prophase I This is when crossing-over may occur, increasing genetic variability

  11. Crossing Over-Prophase I • Genetic Recombination Increasing genetic variation in offspring • Provides new variation • Why do you not look exactly like your siblings?

  12. Metaphase I • Tetrads line up at the equator. • Spindle attaches to the centromeres of each of the chromosomes.

  13. Compare: Which picture is metaphase I of meiosis and which is metaphase of mitosis? What is the difference in the two pictures?

  14. This is called Independent Assortment

  15. Anaphase I • Homologous chromosomessplit & move to opposite poles. • The tetrads split but sisters remain together

  16. Telophase I • Form 2 cells, each of which has either the sister chromatids from the biological mother or from the biological father. In humans, how many chromosomes are present in each cell after meiosis I? How many chromatids?

  17. Figure 11-15 Meiosis Section 11-4 Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

  18. MEIOSIS I ANIMATION http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html Watch an animation of Meiosis I several times, until all stages can be identified and described.

  19. There is NO DNA replication between Meiosis I and Meiosis II. WHY??

  20. MEIOSIS II It is very similar to mitosis

  21. Prophase II • Nuclear membrane disappears • Spindles forms

  22. Chromosomes line up at the equator • Spindle fibers attach to chromosomes Metaphase II

  23. Sister chromatids separate and move to the poles • Spindle fibers shorten Anaphase II

  24. Nuclear membrane reforms • Chromosomes uncoil • Spindle apparatus breaks down Telophase II

  25. Meiosis II Figure 11-17 Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Meiosis II results in four haploid (N) daughter cells. Go to Section:

  26. MEIOSIS II ANIMATION http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html

  27. Oogenesis First meiotic division produces two unequal cells, one oocyte and one polar body Second meiotic division produces four unequal cells, one oocyte and three polar bodies One egg

  28. Spermatogenesis First meiotic division produces two equal cells Second meiotic division produces four equal cells Four sperm

  29. Comparing Mitosis & Meiosis

  30. Mistakes M Mistakes in Meiosis ENTIRE CHROMOSOME MISTAKES • Trisomy • Monosomy • Triploidy • Tetraploidy • We’ll address individual GENE problems later…

  31. Mistakes in Meiosis • What went wrong here? • What might the result be?

  32. Mistiosis • Nondisjunction • Failure of homologues to separate properly

  33. Mistakes in Meiosis • Trisomy –an extra chromosome (3) per gamete Ex: Downs’ Syndrome (TRISOMY 21) Monosomy –ONE LESS CHROMOSOME PER GAMETE • Ex: Turner’s Syndrome

  34. Turner’s Syndrome • Monosomy X (Turners Syndrome) • Missing 1 chromosome = missing 1,000’s of genes!!! • Short neck • short stature • sterile, • Heart, kidney, bone, thyroid problems

  35. Polyploid • In plants • tri & tetra

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