Meiosis

The revision cards for all of your meiosis

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Prophase 1

  • Chromatin condenses and undergoes supercoiling
    • chromsomes shorten and thicken
    • they can absorb stains so can be seen
  • The chromosomes come together in their homologous pairs
    • They form a bivalent
    • Each member of the pair has the same genes at the same loci
    • Each pair has a paternal and maternal chromosome
  • Non sister chromosomes wrap around each other
    • The points they attach at are chiasmata
  • Crossing over  occurs and they swap sections of chromatids with each other
  •  
    • Nucleolus disappears and nuclear envelope disintergrates
    • Spindle forms and
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Prophase 1 cont

(http://www.phschool.com/science/biology_place/biocoach/images/meiosis/mepro1.gif)

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Metaphase 1

  • Bivalents line up along the equator of the spindle
    • the spindle is attached to spindle fibres along the centromere
    • Chiasmata are still present
  • Bivalents are arranged randomly
    • each member of he homologous pair facing opposite poles
  • Chromosomes indipendently segregate

(http://www.phschool.com/science/biology_place/biocoach/images/meiosis/memet1.gif)

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Anaphase 1

  • The homologous chromosomes in each bivalent are pulled apart by the spindle fibres
  • The centromeres do not divide
  • The chiasmata separate and chromatid that have been crossed over remain with the chromatid where they have become newly attached

(http://www.phschool.com/science/biology_place/biocoach/images/meiosis/meana1.gif)

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Telophase 1

  • Two nuclear envelopes form
  • The cell divides by cytokinesis
  • There is brief interphase
  • Chromosomes uncoil

(http://www.phschool.com/science/biology_place/biocoach/images/meiosis/metel1.gif)

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Prophase 2

  • If a nuclear envelope has formed, it is broken again
  • The nucleolus disappears, chromosomes condense and spindles form
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Metaphase 2

  • The chromosomes arrange themselves along the equator of the spindle
  • The chromatids of each chromosome are randomly 

(http://www.utm.utoronto.ca/~w3bio380/picts/supp/supp2/Meiosis_MetaphaseII.jpg)

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Anaphase 2

  • The centromeres divide and chromatids are pulled apart.
  • The chromatids randomly segregate

(http://www.emc.maricopa.edu/faculty/farabee/biobk/meiosis5.gif)

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Telophase 2

  • Nuclear envelope reforms around the haploid daughter nuclei
  • In animals, the two cells now divide to give four haploid cells
  • In plants, a tetrad of four haploid cells is formed

(http://www.phschool.com/science/biology_place/biocoach/images/meiosis/metel2.gif)

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