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  • Created by: Beth
  • Created on: 04-01-14 14:24

Meiosis 1 - Prophase 1

  • The chromatin condenses and undergoes a supercoiling so that the chromosomes shorten and thicken.
  • They can take up stains and be seen with a light microscope.
  • The chromosomes come together in their homologous pairs to form a bilvalent.
  • Each member of the pair has the same number of genes at the same loci.
  • Each pair consists of one maternal and one paternal chromosome.
  • The non sister chromatids wrap around each other and attatch at points called chaismata.
  • They may swap sections of chromatids with one another in a process called crossing over.
  • The nucleolus disappears and the nuclear envelope breaks down.
  • A spindle forms - made of protein microtubules.
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Meiosis 1 - Metaphase 1

  • Bivalents line up across the equator of the spindle, attached to spindle microtubules at the centromeres - The chaismata are still present. 
  • The bivalents are arranged randomly (random assortment) with each member of a homologous pair facing opposite poles.
  • This allows the chromosomes to segregate independently when they are pulled apart in anaphase 1.
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Meiosis 1 - Anaphase 1

  • The homologous chromosomes in each bivalent are pulled by the spindle microtubules to opposite poles.
  • The centromeres do not divide (unlike Mitosis).
  • The chaismata seperate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.
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Meiosis 1 - Telophase 1

  • For most animal cells, nuclear envelopes form around each set of 23 chromosomes and the cell is divided by Cytokinesis. The chromosomes then uncoil and there is a short Interphase.
  • However in most plant cells, there is no Telophase 1, the plant cell goes straight into Anaphase 1 and Meiosis 2.
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Meiosis 2 - Prophase 2

  • If a nuclear envelope has reformed, it breaks down again.
  • The nucleolus dissappears, chromosomes supercoil and spindle forms.
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Meiosis 2 - Metaphase 2

  • The chromosomes arrange themselves on the equator ofthe spindle. They are attached to the spindle microtubules at the centromere.
  • The chromatids of each chromosome are randomly assorted.
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Meiosis 2 - Anaphase 2

  • The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. 
  • The chromatids randomly segregate - become daughter chromosomes.
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Meiosis 2 - Telophase 2

  • Nuclear envelope reforms around the haploid daughter nuclei.
  • In animals, the 2 cells now divide to give 4 haploid cells (cytokinesis).
  • In plants, a tetrad of 4 haploid cells is formed (no cytokinesis).
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