Significance of meiosis
- Created by: une-meduse
- Created on: 17-05-13 21:01
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- HOW DOES MEIOSIS AND FERTILISATION LEAD TO VARIATION
- CROSSING OVER
- 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
- 6 - The chiasmata remain in place during metaphase, they hold the maternal and paternal chromosomes in place on the spindle
- 5 - This produces new combinations of alleles on the chromatids
- 4 - This is called crossing over
- 3 -This leads to similar sections of non sister chromatids being swapped over, these contain the same genes but different alleles
- 2 - Chromosomes may break at these points, the broken ends rejoin to the end of the non sister chromatids in the same bivalent
- 1 - non sister chromatids wrap around each other very tightly and attach at chiasmata
- Occurs during prophase 1, on average 2 and 3 cross over events occur on each pair of human chromosomes
- RE - ASSORTMENT OF CHROMOSOMES
- 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
- From this, theoretically one individual could produce 2n genetically different gametes (where n=the haploid number of chromosomes
- Each gamete acquires a different mixture of maternal and paternal chromosomes.
- And the subsequent segregation into two daughter nuclei at anaphase 1
- The consequence of the random distribution of maternal and paternal chromosomes on the spindle equator at metaphase 1
- RE - ASSORTMENT OF CHROMATIDS
- 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
- Because of crossing over, the sister chromatids are no longer genetically identical
- This is the result of the random distribution on the spindle equator of the sister chromatids at metaphase 2
- FERTILISATION
- 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
- There are around 300 million spermatoza, all genetically different, and any one of them can fertilise the secondary oocyte
- In humans one ovum (secondary oocyte) is usually released from an ovary at a time
- MUTATION
- 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
- Chromosome mutations may also occur, if it does then the mutated gene will also be present in every cell
- 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
- CROSSING OVER
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