Meiosis

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  • Meiosis
    • after interphase there are two divisions: MEIOSIS I & MEIOSIS II
    • MEIOSIS I
      • Prophase I
        • CHROMATIN condenses to form chromosomes
        • nuclear membrane & nucleolus breaks down and disappears
        • paternal and maternal chromosomes come together in HOMOLOGOUS PAIRS .
          • the pairing of chromosomes is called SYNAPSIS
            • each chromosome in  a homologous pair forms a BIVALENT
              • Bivalent - the association of the two chromosomes of a homologous pair at prophase I in meiosis
              • the chromatids's homologous pairs touch at CHIASMATA
                • 'crossing over' occurs
                  • a segment of DNA from one chromatid is exchanged between the homologous pairs in a bivalent
                    • centrioles produce proteins for microtubules to form spindle
                  • v. important source of GENETIC VARIATION - the two chromatids of the chromosomes are NO LONGER IDENTICAL
                    • produces new combination of alleles
                      • four haploid gametes with different genetic composition
                    • a segment of DNA from one chromatid is exchanged between the homologous pairs in a bivalent
                      • centrioles produce proteins for microtubules to form spindle
        • Metaphase I
          • Bivalents arrange (RANDOMLY) at equator of cell
            • leads to INDEPENDENT ASSORTMENT
              • important source of variation
            • spindle fibres attach at centromeres of each chromosome
          • Anaphase I
            • spindle fibres contract, bivalents seperate
              • chromosomes pulled to opposite ends of poles
                • Centromeres have NOT SPLIT so chromosomes are still DOUBLE structures
                • [Each pole only receives one of each homologous pair]
            • Telophase I
              • Cytokinesis occurs and TWO new haploid cells are produced
    • MEIOSIS II
      • Prophase II
        • centrioles separate and form spindle at right angles to the spindle used in meiosis I
        • Metaphase II
          • chromosomes line up on the equator.
            • spindle fibres attach at the centromeres
          • Anaphase II
            • spindle fibres contract
              • centromeres split and chromosomes pulled to opposite poles of the cell.
                • they are NO LONGER DOUBLE STRUCTURES
            • Telophase II
              • chromosomes uncoil to form chromatin & nuclear envelope reforms
                • cytokinesis occurs and each cell produces two cells
    • DNA replication only happens BEFORE MEIOSIS I
    • Diagram showing haploid no.

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