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Meiosis

Meiosis Meiosis is often called reduction division because it cuts the number of chromosomes in half . Example: humans from 46 to 23. Meiosis occurs only in organisms that reproduce sexually , in the testes of males, and the ovaries of females.

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Meiosis

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  1. Meiosis Meiosis is often called reduction divisionbecause it cuts the number of chromosomes in half. Example: humans from 46 to 23. Meiosis occurs only in organisms that reproduce sexually, in the testes of males, and the ovaries of females.

  2. Testes and ovaries are packed with germ cells that start the meiotic process. In the end sex cells, or gametes are produced: sperm in males, eggs, or ova, in females, or spores in SOME plants. (Most plants make sperm and egg.)

  3. Meiosis: The Formation of Gametes Goal of Meiosis: Produce haploid sex cells (gametes) Sperm + Egg (Ovum) = Zygote (Haploid) + (Haploid) = (Diploid) 23 + 23 = 46

  4. Unlike mitosis, meiosis produces 4 cells, is not a cycle, and makes unique cells, instead of identical ones.

  5. Meiosis occurs in two divisions: Meiosis I:Reduction Division;reduce the number of chromosomes in half DiploidParent Cell ---------HaploidDaughter Cells Meiosis II:Equatorial Division;splitting of replicated DNA HaploidParent Cells -------HaploidDaughter Cells

  6. Meiosis contains 2 parts: Meiosis I and MeiosisII. Similar to mitosis, meiosis contains PMAT in each Meiosis part. During PMAT1 the chromosomes are randomly assorted. Meiosis II is just like mitosis except it deals with haploid cells (represented by 1n), instead of diploid cells.

  7. Prophase I: DNA (chromatin) begins to condense to form chromosomes Nucleolus and nuclear membrane begin to disappear Centrioles begin migration to opposite ends of the cell Spindlefibers form from the centrioles Synapsis Occurs: Pairing of Homologous Chromosomes TetradsFormed: 4 chromatid legs, may crossoverand exchange pieces of DNA

  8. Prophase I Crossover

  9. Crossing Over During Synapsis

  10. Crossing-over “shuffles” the DNA on the chromosomes in many different unique combinations. In other words, parents will produce many different “variations” of gametes in which after fertilization, their offspring will be sometimes similar but different !

  11. Metaphase I: Homologous pairs (tetrads) attach to spindle and move to the equator of the cell.

  12. Anaphase I: Events: 1.Tetrads separate into individual chromosomes (2 chromatids each) and move to the poles. There are now 23 chromosomes at each end of the cell, or halfas many as when meiosis started. 2. Spindle fibers begin to break down.

  13. Telophase I: Chromosomes reach the oppositeendsof the cell Daughter cells are formed by the pinching of the cell membrane (cleavage furrow)

  14. Meiosis I

  15. Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II) Meiosis II is essentially the same as mitosis, except each cell has half as many chromosomes, and is called haploid. The phases occur in the same order, and contain the same events. Since meiosis II begins with two cells, it ends with four cells. All four cells are genetically different from the others that were produced due to the crossing over that occurred in Prophase I

  16. Prophase II: Centrioles begin migration to opposite ends of the cell New spindle fibers form from the centrioles

  17. Metaphase II: Chromosomes attach to spindle and move to the equator of the cell.

  18. Anaphase II: Chromosomes are split and sister chromatids move to opposite ends

  19. Telophase II: Sister chromatids reach the opposite ends Nucleus and nucleolus begin to re-appear Daughter cells are formed by the pinching of the cell membrane (cleavage furrow)

  20. Meiosis II

  21. Meiosis Animation Meiosis Video

  22. Meiosis in Males & Females

  23. Spermatogenesis Spermatogenesis begins at puberty in males. Humans release 300-400 million per ejaculation. In humans spermatogenesis is a non-stop process from puberty until death, and each germ cell results in four viable sperm cells. Cytokinesis divides the sperm cells equally. Mitosiscontinually produces new germ cells to replace ones that have gone through Meiosis.

  24. Oogenesis Oogenesisbegins during embryonic development and continues until shortly after birth, when all germ cells have gone through Prophase I. The cells then go dormant, until puberty, when one cell per month continues to Metaphase II. This process stops at menopause.

  25. Anaphase II and Telophase II only occur if fertilization by sperm occurs. Humans will produce only 3-4 hundred eggs in her lifetime. During meiosis cytokinesis favors one cell, so each germ cell yields one viable egg. The other three cells are too tiny to function and are called polar bodies.

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