Meiosis

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  • MEIOSIS 1
    • Meiosis is a reduction division, the resulting daughter cells have half the original number of chromosomes
      • They are known as haploid and can be used for sexual reproduction
        • Each parent produces reproductive cells called gametes
          • Gametes fuse together at fertilisation to produce a zygote
    • (Interphase)
      • PROPHASE 1
        • METAPHASE 1
          • ANAPHASE 1
            • TELOPHASE 1
              • In most  animal cells two new nuclear envelopes form
                • One around each set of chromosomes at each pole and the cell divides by cytokineses
                  • There is usually a brief interphase and the chromosomes uncoil
                    • In plant cells the cell goes straight from anaphase 1 into meiosis 2 (mostly)
              • MEIOSIS 2
                • This division is at right angles to meiosis 1
                • PROPHASE 2
                  • METAPHASE 2
                    • The chromosomes arrange themselves on the equator of the spindle
                      • The  chromosomes   are attached to the spindle microtubules at the centromeres
                        • The chromatids of each chromosome  are randomly assorted
                    • ANAPHASE 2
                      • The centromeres divide
                        • The chromatids are pulled to opposite poles by the spindle fibres
                          • The chromatids randomly segregate
                      • TELOPHASE 2
                        • Nuclear envelopes reform around the haploid daughter nuclei
                          • In animals the two new cells now divide and give four haploid cells
                            • In plants a tetrad of four haploid cells is formed
            • The homologous  chromosomes in each bivalent are pulled by the spindle microtubules to opposite poles
              • Centromeres (The point on a chromosome by which it is attached to a spindle fiber during cell division) do not divide
                • The chiasmata (the points at which non-sister chromatids with a bivalent join) separate
                  • Lengths of chromatid (The two threadlike strands) that have been crossed over remain with the chromatid that they have become attatched
                    • Chromatid - Each of the two threadlike strands into which a chromosome divides longitudinally during cell division. Each contains a double helix of DNA.
          • Bivalents (the pair of joined homologous chromosomes) line up across equator  of spindle and attach at centromeres
            • Bivalents (the joined chromosomes) arrange randomly
              • Each member of the homologous pair faces opposite poles
        • Chromatin (the mass of genetic material composed of DNA and proteins)  condenses and supercoils
          • Chromosomes come together in homologous pairs to form a bivalent
      • At interphase before mitosis 1 the DNA replicates
        • As a result each chromosome consists of two identical sister chromatids (half the chromosome) joined at the centromere
  • If a nuclear envelope has reformed, it breaks down again
    • PROPHASE 2
      • METAPHASE 2
        • The chromosomes arrange themselves on the equator of the spindle
          • The  chromosomes   are attached to the spindle microtubules at the centromeres
            • The chromatids of each chromosome  are randomly assorted
        • ANAPHASE 2
          • The centromeres divide
            • The chromatids are pulled to opposite poles by the spindle fibres
              • The chromatids randomly segregate
          • TELOPHASE 2
            • Nuclear envelopes reform around the haploid daughter nuclei
              • In animals the two new cells now divide and give four haploid cells
                • In plants a tetrad of four haploid cells is formed
    • The nuclolus disappears, chromosomescondense and spindles form

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