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Explanation slides Mitosis and Meiosis

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Explanation slides Mitosis and Meiosis

  1. 1. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Mitosis and Meiosis This PowerPoint file contains a number of slides that may be useful for teaching of genetics concepts. You may use these slides and their contents for non-commercial educational purposes. This presentation contains diagrams of: • Mitosis • Meiosis • Meiotic non-disjunction
  2. 2. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk What is the purpose of mitosis? Cell division Products genetically identical Growth of organism
  3. 3. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  4. 4. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.6 ©Scion Publishing Ltd The stages of mitosis See next slides for individual stages
  5. 5. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  6. 6. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  7. 7. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  8. 8. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  9. 9. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Meiosis • Function Reduction division (23 chromosomes per gamete) reassortment of genes by: • crossing-over • independent segregation of chromosomes • Mechanism Each homologue (e.g. “chromosome 7”) replicates to give two sister chromatids Homologues pair (e.g. maternal chromosome 7 and paternal chromosome 7) Exchange of material between non-sister chromatids: crossing-over, recombination Chiasmata (visible cytologically) are the physical manifestations of crossing-over
  10. 10. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk A homologous pair of parental chromosomes (e.g. chromosome 7) In meiosis I each chromosome duplicates producing two sister chromatids Crossing-over (Recombination) Gene re-assortment by crossing-over meiosis II
  11. 11. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Each spermatogonium in the testis at age 15 is the result of 30 previous cell divisions This spermatogonium maintains the stock of spermatogonia and continues to divide Four spermatozoa Every 16 days from puberty At the age of 25: 310 cell divisions have had to occur to produce a particular sperm. The number of cell divisions required to produce a human sperm Four spermatozoa
  12. 12. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk MEIOSIS I Each spermatogonium in testis at age 15 is result of 30 previous mitotic cell divisions Pool of spermatogonia maintained and continues to divide 4 spermatozoa (Every 16 days from puberty) At the age of 25: 310 cell divisions have had to occur to produce a particular sperm. The number of cell divisions required to produce a human sperm primary spermatocyte SG SG SG SC SC secondary spermatocytes MEIOSIS II SC 4 spermatids differentiation MITOSIS SG
  13. 13. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk 22 mitotic cell divisions by 5 months gestation to make a stock of 2,600,000 oocytes Each month one is ovulated MEIOSIS I completed at ovulation Polar body Meiosis II completed at fertilisation 2nd polar body Zygote The number of cell divisions required to produce a human egg cell
  14. 14. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk The stock of oocytes is ready by 5 months gestation. Each remains in maturation arrest at the crossing-over stage until ovulation Each month one is ovulated Meiosis I not completed until ovulationPolar body Meiosis II not completed until fertilisation 2nd polar body Zygote Oocytes, time and the completion of meiosis There may be a lengthy interval between onset and completion of meiosis (up to 50 years later) Accumulating effects on the primary oocyte during this phase may damage the cell’s spindle formation and repair mechanisms predisposing to non-disjunction.
  15. 15. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  16. 16. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.7 ©Scion Publishing Ltd The stages of meiosis. Meiosis is used only for the production of sperm and eggs. It consists of two successive cell divisions, producing four daughter cells (although in oogenesis only one of these develops into a mature oocyte; the others form the polar bodies). Meiosis has two main functions: to reduce the chromosome number in the gamete to 23, and to ensure that every gamete is genetically unique.
  17. 17. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  18. 18. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  19. 19. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.8 © Scion Publishing Ltd Examples of chromosomes during meiosis. (a)Two cells from a testicular biopsy showing chromosomes during prophase I of male meiosis. Each of the 23 structures is a bivalent, consisting of two homologous chromosomes, each having two chromatids. Note the end-to-end pairing of the X and Y chromosomes. (b)A bivalent seen in meiosis in an amphibian, which has large chromosomes that make the four- stranded structure clear.
  20. 20. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.12 © Scion Publishing Ltd The effects of non-disjunction in meiosis. The non-disjunction involves only the single pair of chromosomes (meiosis I) or the single chromosome (meiosis II) shown; all the other chromosomes (not shown) disjoin and segregate normally.
  21. 21. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.17 ©Scion Publishing Ltd Possible ways the chromosomes could segregate in the first meiotic division. During prophase 1, matching chromosome segments pair, resulting in a cross-shaped tetravalent containing the normal and translocated copies of chromosomes 1 and 22. At anaphase 1 they pull apart, and the diagram shows various ways this could happen. The gamete that gave rise to Baby Elliot is circled. Other more complex segregation patterns (3:1 segregation) are also possible.
  22. 22. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Fig. 2.21 ©Scion Publishing Ltd During meiosis I matching chromosome segments pair. If one chromosome has an inversion compared to its homolog, they usually form a looped structure.
  23. 23. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk
  24. 24. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Normal monosomic gametes Normal meiosis Reduction division MEIOSIS I MEIOSIS II Results of crossing-over not shown Replicate DNA
  25. 25. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk MEIOSIS I MEIOSIS II Results of crossing-over not shown Replicate DNA Nondisjunction during meiosis I Non-disjunction Disomic gametes Nullisomic gametes
  26. 26. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk MEIOSIS I MEIOSIS II Results of crossing-over not shown Replicate DNA Nondisjunction during meiosis II Non-disjunction Disomic Nullisomic Monosomic Monosomic gametes
  27. 27. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Parental origin of meiotic error leading to aneuploidy Chromosome abnormality Paternal (%) Maternal (%) Trisomy 21 (Down) 15 85 Trisomy 18 (Edwards) 10 90 Trisomy 13 (Patau) 15 85 45,X (Turner) 80 20 47,XXX 5 95 47,XXY 45 55 47,XYY 100 0
  28. 28. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk New mutations: increase with paternal age 0 1 2 3 4 5 24 29 34 39 44 47 Paternal age Relativefrequency Marfan Achondroplasia Higher mutation rates in males are likely to be related to the greater number of germ cell divisions
  29. 29. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Meiosis Animation from Tokyo Medical University Genetics Study Group Hironao NUMABE, M.D
  30. 30. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Non-disjunction in meiosis I resulting in trisomy 21 Down syndrome Animation from Tokyo Medical University Genetics Study Group Hironao NUMABE, M.D
  31. 31. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Normal disomy Mitosis Non-disjunction Normal disomy Trisomy Monosomy (lethal to cell) Somatic mosaicism (eg trisomy 21) as a result of mitotic non-disjunction
  32. 32. © 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare www.geneticseducation.nhs.uk Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1) Trisomy Monosomy (lethal)

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