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1. Meiosis (Core) Stephen Taylor 4.2 Meiosis (Core) 1 http://sciencevideos.wordpress.com

2. Meiosis A reduction division of nuclei to form haploid gametes. 4.2 Meiosis (Core) 2 http://sciencevideos.wordpress.com

3. Meiosis A reduction division of nuclei to form haploid gametes. Somatic cell nuclei (body cells) are diploid (2n). They contain an homologous pair of each chromosome. Humans have 23 pairs of chromomes(n=23). A diploid (2n) cell therefore contains 2x23 = 46 chromosomes. In this example cell we see two pairs of homologous chromosomes. It is a diploid cell of n=2. 4.2 Meiosis (Core) 3 http://sciencevideos.wordpress.com

4. Meiosis A reduction division of nuclei to form haploid gametes. Somatic cell nuclei (body cells) are diploid (2n). They contain an homologous pair of each chromosome. Humans have 23 pairs of chromomes(n=23). A diploid (2n) cell therefore contains 2x23 = 46 chromosomes. These chromosomes replicate in the S-phase of interphase to make pairs of sister chromatids, joined at the centromere. Each chromosome has replicated. It is still diploid but now they are know as pairs of sister chromatids. 4.2 Meiosis (Core) 4 http://sciencevideos.wordpress.com

5. Meiosis A reduction division of nuclei to form haploid gametes. Somatic cell nuclei (body cells) are diploid (2n). They contain an homologous pair of each chromosome. Humans have 23 pairs of chromomes(n=23). A diploid (2n) cell therefore contains 2x23 = 46 chromosomes. These chromosomes replicate in the S-phase of interphase to make pairs of sister chromatids, joined at the centromere. Meiosis is a process of two divisions. Meiosis I separates the homologous pairs – this is the reduction division. The homologous pairs have separated but the sister chromatids remain attached. 4.2 Meiosis (Core) 5 http://sciencevideos.wordpress.com

6. Meiosis A reduction division of nuclei to form haploid gametes. Somatic cell nuclei (body cells) are diploid (2n). They contain an homologous pair of each chromosome. Humans have 23 pairs of chromomes(n=23). A diploid (2n) cell therefore contains 2x23 = 46 chromosomes. These chromosomes replicate in the S-phase of interphase to make pairs of sister chromatids, joined at the centromere. Meiosis is a process of two divisions. Meiosis I separates the homologous pairs – this is the reduction division. Meiosis II separates the sister chromatids. The end product of meiosis from a single somatic diploid cell is four haploid gametes. 4.2 Meiosis (Core) 6 http://sciencevideos.wordpress.com

7. Homologous Chromosomes same structure Somatic cell nuclei (body cells) are diploid (2n). They contain an homologous pair of each chromosome. One of the pair is paternal – inherited from the father. The other is maternal – inherited from the mother. Homologous chromosomes are the same size and structure. They carry the same genes at the same loci. Alleles carried at each locus may vary. 22 of the human chromosome pairs are homologous. The other pair, the sex chromosomes, are non-homologous. Banding pattern Centromere position size 4.2 Meiosis (Core) 7 http://sciencevideos.wordpress.com

8. Interphase In preparation for the reduction division of meiosis, the chromosomes replicate. This occurs in the Synthesis phase (S-phase) of meiosis, through DNA Replication. Each single chromosome becomes a pair of sister chromatids, physically joined at the equator. The centromere is a junction between the sister chromatids. 4.2 Meiosis (Core) 8 http://sciencevideos.wordpress.com

9. Prophase I The homologous chromosomes associate with each other. The nuclear membrane breaks down and centrioles migrate to the poles. 4.2 Meiosis (Core) 9 http://sciencevideos.wordpress.com

10. Prophase I The homologous chromosomes associate with each other. Crossing-over between non-sister chromatids can take place. The nuclear membrane breaks down and centrioles migrate to the poles. This results in recombination of alleles and is a source of genetic variation in gametes. 4.2 Meiosis (Core) 10 http://sciencevideos.wordpress.com

11. Metaphase I The bivalents (homologous pairs) line up at the equator. Random orientation of these homologous pairs (the way they face) leads to massive genetic variation in the gametes. There are 223 possible orientations in humans – well over 8 million! 4.2 Meiosis (Core) 11 http://sciencevideos.wordpress.com

12. Anaphase I Spindle fibres contract. Homologous pairs are separated and pulled to opposing poles. This is the reduction division. Non-disjunction here will affect the chromosome number of all four gametes. 4.2 Meiosis (Core) 12 http://sciencevideos.wordpress.com

13. Telophase I New nuclei form and the cytoplasm begins to divide by cytokinesis. The nuclei are no longer diploid. They each contain one pair of sister chromatids for each of the species’ chromosomes. If crossing-over and recombination has occurred then the sister chromatids will not be exact copies. 4.2 Meiosis (Core) 13 http://sciencevideos.wordpress.com

14. Interphase There is no Synthesis phase in Interphase II. 4.2 Meiosis (Core) 14 http://sciencevideos.wordpress.com

15. Prophase II The nuclei break down. No crossing-over occurs. 4.2 Meiosis (Core) 15 http://sciencevideos.wordpress.com

16. Metaphase II Pairs of sister chromatids align at the equator. Spindle fibres form and attach at the centromeres. Random orientation again contributes to variation in the gametes, though not to such an extent as in metaphase I. This is because there is only a difference between chromatids where crossing-over has taken place. 4.2 Meiosis (Core) 16 http://sciencevideos.wordpress.com

17. Anaphase II Spindle fibres contract and the centromeres are broken. The pairs of sister chromatids are pulled to opposing poles. Non-disjunction here will lead to two gametes containing the wrong chromosome number. 4.2 Meiosis (Core) 17 http://sciencevideos.wordpress.com

18. Telophase II New haploid nuclei are formed. Cytokinesis begins, splitting the cells. The end result of meiosis is four haploid gamete cells. Fertilisation of these haploid gametes will produce a diploid zygote. 4.2 Meiosis (Core) 18 http://sciencevideos.wordpress.com

19. Outline the Process of Meiosis 4.2 Meiosis (Core) 19 http://sciencevideos.wordpress.com

20. Outline the Process of Meiosis 4.2 Meiosis (Core) 20 http://sciencevideos.wordpress.com

21. Outline the Process of Meiosis 4.2 Meiosis (Core) 21 http://sciencevideos.wordpress.com

22. Non-disjunction Normal meiosis produces four haploid gametes. Meiosis I Reduction division 4.2 Meiosis (Core) 22 http://sciencevideos.wordpress.com

23. Non-disjunction Normal meiosis produces four haploid gametes. Meiosis I Reduction division Meiosis II Meiosis II Separation of sister chromatids n n n n 4.2 Meiosis (Core) 23 http://sciencevideos.wordpress.com

24. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. 4.2 Meiosis (Core) 24 http://sciencevideos.wordpress.com

25. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division 4.2 Meiosis (Core) 25 http://sciencevideos.wordpress.com

26. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division 4.2 Meiosis (Core) 26 http://sciencevideos.wordpress.com

27. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division Meiosis II Meiosis II Separation of sister chromatids n-1 n-1 n+1 n+1 4.2 Meiosis (Core) 27 http://sciencevideos.wordpress.com

28. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division 4.2 Meiosis (Core) 28 http://sciencevideos.wordpress.com

29. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division 4.2 Meiosis (Core) 29 http://sciencevideos.wordpress.com

30. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction can occur anaphase I or anaphase II. In anaphase, a chromosome or pair of sister chromatids can be pulled to the wrong pole. Meiosis I Reduction division Meiosis II Meiosis II Separation of sister chromatids n+1 n n n-1 4.2 Meiosis (Core) 30 http://sciencevideos.wordpress.com

31. Non-disjunction Non-disjunction leads to changes in chromosome number. Non-disjunction in Meiosis II Non-disjunction in Meiosis I Animation from BioStudio: http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.htm Animation from BioStudio: http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm Half of gametes affected. All gametes affected. 4.2 Meiosis (Core) 31 http://sciencevideos.wordpress.com

32. Non-disjunction & Trisomy Disorders Non-disjunction produces gametes with an abnormal number of chromosomes. Fertilisation adds the homologous chromosomes… n n+1 4.2 Meiosis (Core) 32 http://sciencevideos.wordpress.com

33. Non-disjunction & Trisomy Disorders Non-disjunction produces gametes with an abnormal number of chromosomes. Fertilisation adds the homologous chromosomes… n n+1 Resulting in a zygote and somatic cells with a trisomy of one chromosome. This can be fatal or cause disorders. 4.2 Meiosis (Core) 33 http://sciencevideos.wordpress.com

34. Down Syndrome Trisomy 21 Non-disjunction in anaphase I or II leads to an extra copy of chromosome 21 in the gamete (usually the egg). Fertilisation by a sperm adds the homologous chromosome 21, leading to trisomy. Down Syndrome has very distinctive physical characteristics and patients have a slightly shortened life expectancy, learning difficulties and a generally happy nature. Computer-enhanced karyogram (adapted): http://www.smd.lt/bureliu_upload/genetika/karyogram.gif NSW Down Syndrome Video: http://www.youtube.com/watch?v=tDjnNDRP_2o 4.2 Meiosis (Core) 34 http://sciencevideos.wordpress.com

35. Maternal Age and Down Syndrome Risk of Down Syndrome is strongly correlated with maternal age. Use this graph to generate your own data analysis questions. http://ibis.health.utah.gov/indicator/view/BrthDefDownSyn.BrthPrevAge.html 4.2 Meiosis (Core) 35 http://sciencevideos.wordpress.com

37. What are the ethical issues regarding pre-natal testing and abortion?

38. “Forewarned is forearmed,” Discuss. 4.2 Meiosis (Core) 36 http://sciencevideos.wordpress.com

39. Karyotyping Pre-natal test used to check for gender and trisomy disorders. Extract fetal cells by amniocentesis or chorionic villus sampling. Culture cells and stimulate mitosis. Stop division in metaphase. Banding pattern Take a photograph under the light microscope or scan with a computer. Arrange chromosomes in homologous pairs based on size, banding patterns and centromere positions. 6. Check for gender (XX or XY) or trisomy disorders. Centromere position size Karyotype animation: http://learn.genetics.utah.edu/content/begin/traits/karyotype/ Step-by-step animation: http://www.mwit.ac.th/~bio/assets/karyotype_mutation.swf 4.2 Meiosis (Core) 37 http://sciencevideos.wordpress.com

41. Via abdomen

43. Via cervix or abdomen

44. Risk of miscarriage: up to 1 in 100Find out more: what are the differences between 12-week and 15-week fetuses and how might this affect the mother’s decision making? 4.2 Meiosis (Core) 38 http://sciencevideos.wordpress.com

45. Nuchal Translucency Scans Non-syllabus Amniocentesis and CVS cell extraction run a risk of miscarriage. The non-invasive nuchal translucency scan allows doctors to determine whether there is need for a karyotype. If the nuchal fold is large and fluid-filled, it can be an indicator of Down Syndrome. If it is normal, Down Syndrome is very unlikely. Nuchal translucency scan – is it positive or negative? http://www.youtube.com/watch?v=Jt3cX2vuMyM Find out more about Down Syndrome and screening from the UK’s NHS: http://www.nhs.uk/news/2008/11November/Pages/DownssyndromeQA.aspx Can you see the difference between these two scans? What advice would the doctor give? http://www.babycentre.co.uk/pregnancy/antenatalhealth/scans/nuchalscan/ 4.2 Meiosis (Core) 39 http://sciencevideos.wordpress.com

46. Explain how inheritance of chromosome 21 can lead to Down Syndrome. 3 marks 4.2 Meiosis (Core) 40 http://sciencevideos.wordpress.com

48. This is when a homologous pair of chromosome 21 fail to separate (anaphase I) or a pair of sister 21 chromatids fail to separate (anaphase II).

49. It can occur in anaphase I or II.

51. Analyse this Karyotype Gender: Trisomy: Computer-enhanced karyogram: http://www.smd.lt/bureliu_upload/genetika/karyogram.gif 4.2 Meiosis (Core) 42 http://sciencevideos.wordpress.com

53. Analyse this Karyotype Gender: Trisomy: Karyogram from Tokyo Medical University: http://www.tokyo-med.ac.jp/genet/cki-e.htm 4.2 Meiosis (Core) 44 http://sciencevideos.wordpress.com

55. Down SyndromeKaryogram from Tokyo Medical University: http://www.tokyo-med.ac.jp/genet/cki-e.htm 4.2 Meiosis (Core) 45 http://sciencevideos.wordpress.com

56. Analyse this Karyotype Gender: Trisomy: Karyogram from Tokyo Medical University: http://www.tokyo-med.ac.jp/genet/cki-e.htm 4.2 Meiosis (Core) 46 http://sciencevideos.wordpress.com

58. Edwards SyndromeKaryogram from Tokyo Medical University: http://www.tokyo-med.ac.jp/genet/cki-e.htm 4.2 Meiosis (Core) 47 http://sciencevideos.wordpress.com

59. Analyse this Karyotype Gender: Trisomy: Karyogram from Tokyo Medical University: http://www.tokyo-med.ac.jp/genet/cki-e.htm 4.2 Meiosis (Core) 48 http://sciencevideos.wordpress.com

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