Significance of meiosis

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      • Occurs during prophase 1, on average 2 and 3 cross over events occur on each pair of human chromosomes
        • 1 - non sister chromatids wrap around each other very tightly and attach at  chiasmata
          • 2 - Chromosomes may break at these points, the broken ends rejoin to the end of the non sister chromatids in the same bivalent
            • 3 -This leads to  similar sections of non sister chromatids being swapped over, these contain the same genes but different alleles
              • 4 - This is called crossing over
                • 5 - This produces new combinations of alleles on the chromatids
                  • 6 - The chiasmata remain in place during metaphase, they hold the maternal and paternal chromosomes in place on the spindle
                    • 7 - Holding the homologous pairs on the spindle equator ensures that when segregation occours at anaphase 1, one member of each pair goes to each pole
      • The consequence of the random distribution of maternal and paternal chromosomes on the spindle equator at metaphase 1
        • And the subsequent segregation into two daughter nuclei at anaphase 1
          • Each gamete acquires a different mixture of maternal and paternal chromosomes.
            • From this, theoretically one individual could produce 2n genetically different gametes (where n=the haploid number of chromosomes
              • The actual number is actually much greater than this because of crossing over, and the subsequent genetic recombination during prophase 1
      • This is the result of the random distribution on the spindle equator of the sister chromatids at metaphase 2
        • Because of crossing over, the sister chromatids are no longer genetically identical
          • How they align at metaphase 2 determines how they seperate at anaphase 2
      • In humans one ovum (secondary oocyte) is usually released from an ovary at a time
        • There are around 300 million spermatoza, all genetically different, and any one of them can fertilise the secondary oocyte
          • Whichever one fertiloses the secondary oocyte, genetic material from the two unrelated individuals is combined to make a zygote
      • DNA mutation may also occur during interphase when DNA replicates. This is not peculiar to meiosis as it can also occur in miTOsis or binary fission
        • Chromosome mutations may also occur, if it does then the mutated gene will also be present in every cell
          • Inversion - a section of chromosome turns through 180 degrees
          • Deletion - a part is lost
          • Translocation - a piece of one chromosome becomes attached to another
          • Non-Disjunction - homologous chromosomes fail to seperate in meiosis 1 or chromatids fail to seperate at meiosis 2


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