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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