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1. 10.1 Meiosis HL 1 http://sciencevideos.wordpress.com

2. An homologous pair of chromosomes… 10.1 Meiosis HL 2 http://sciencevideos.wordpress.com

3. An homologous pair of chromosomes… …replicates during S-phase of interphase… 10.1 Meiosis HL 3 http://sciencevideos.wordpress.com

4. An homologous pair of chromosomes… …replicates during S-phase of interphase… centromere sister chromatids …giving two pairs of sister chromatids, each joined at the centromere. 10.1 Meiosis HL 4 http://sciencevideos.wordpress.com

5. The homologous pair associates during prophase I, through synapsis… 10.1 Meiosis HL 5 http://sciencevideos.wordpress.com

6. The homologous pair associates during prophase I, through synapsis… …making a bivalent. 10.1 Meiosis HL 6 http://sciencevideos.wordpress.com

7. Crossing-over might take place between non-sister chromatids in prophase I… 10.1 Meiosis HL 7 http://sciencevideos.wordpress.com

8. Crossing-over might take place between non-sister chromatids in prophase I… …leading to recombination of alleles. 10.1 Meiosis HL 8 http://sciencevideos.wordpress.com

9. In anaphase I, the homologous pair is separated but the sister chromatids remain attached. This is the reduction division. 10.1 Meiosis HL 9 http://sciencevideos.wordpress.com

10. Check your language. This image shows… Four separate chromosomes. A bivalent. One pair of sister chromatids. Non-disjunction. 10.1 Meiosis HL 10 http://sciencevideos.wordpress.com

11. Check your language. This image shows… Four separate chromosomes. A bivalent. One pair of sister chromatids. Non-disjunction. 10.1 Meiosis HL 11 http://sciencevideos.wordpress.com

12. Check your language. This image shows… Two separate chromosomes. A bivalent. One pair of sister chromatids. Crossing-over. 10.1 Meiosis HL 12 http://sciencevideos.wordpress.com

13. Check your language. This image shows… Two separate chromosomes. A bivalent. One pair of sister chromatids. Crossing-over. 10.1 Meiosis HL 13 http://sciencevideos.wordpress.com

14. Check your language. This image shows… Two separate chromosomes. A bivalent. One pair of sister chromatids. Homologous chromosomes. 10.1 Meiosis HL 14 http://sciencevideos.wordpress.com

15. Check your language. This image shows… Two separate chromosomes. A bivalent. One pair of sister chromatids. Homologous chromosomes. 10.1 Meiosis HL 15 http://sciencevideos.wordpress.com

16. Check your language. This image shows… 8 separate chromosomes. Two bivalents. Two pairs of sister chromatids. Two homologous chromosomes. 10.1 Meiosis HL 16 http://sciencevideos.wordpress.com

17. Check your language. This image shows… 8 separate chromosomes. Two bivalents. Two pairs of sister chromatids. Two homologous chromosomes. 10.1 Meiosis HL 17 http://sciencevideos.wordpress.com

18. Meiosis Is a reduction division from diploid somatic cells (2n) to produce haploid gametes (n). The reduction is in the chromosome number in each nucleus. 10.1 Meiosis HL 18 http://sciencevideos.wordpress.com

19. Interphase In the S-phase of the interphase before meiosis begins, DNA replication takes place. Chromosomes are replicated and these copies are attached to each other at the centromere. The attached chromosome and its copy are known as sister chromatids. Following S-phase, further growth and preparation take place for meiosis. 10.1 Meiosis HL 19 http://sciencevideos.wordpress.com

21. Which have replicated to form two pairs of sister chromatids.10.1 Meiosis HL 20 http://sciencevideos.wordpress.com

23. Which have replicated to form two pairs of sister chromatids.10.1 Meiosis HL 21 http://sciencevideos.wordpress.com

24. Crossing-Over Increases genetic variation through recombination of linked alleles. Synapsis Homologous chromosomes associate Chiasma Formation Neighbouring non-sister chromatids are cut at the same point. A Holliday junction forms as the DNA of the cut sections attach to the open end of the opposite non-sister chromatid. Recombination As a result, alleles are swapped between non-sister chromatids. 10.1 Meiosis HL 22 http://sciencevideos.wordpress.com

25. Crossing-Over Increases genetic variation through recombination of linked alleles. Crossing over leads to more variation in gametes. This is the standard notation for writing genotypes of alleles on linked genes. More of this later when we study 10.2 Dihybrid crosses and gene linkage. 10.1 Meiosis HL 23 http://sciencevideos.wordpress.com

26. Prophase I The homologous chromosomes associate with each other. Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes. Crossing-over is more likely to occur between genes which are further apart. In this example, there will be more recombination between D and E than between C and D. During prophase, the nuclear membrane also breaks down and the centrioles migrate to the poles of the cell. 10.1 Meiosis HL 24 http://sciencevideos.wordpress.com

27. Metaphase I The bivalents line up at the equator. Random orientation occurs and is a significant source of genetic variation. There are 2n possible orientations in metaphase I and II. That is 223 in humans – or 8,388,068 different combinations in gametes! 10.1 Meiosis HL 25 http://sciencevideos.wordpress.com

28. 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. 10.1 Meiosis HL 26 http://sciencevideos.wordpress.com

29. 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. 10.1 Meiosis HL 27 http://sciencevideos.wordpress.com

30. Interphase There is no Synthesis phase in Interphase II. 10.1 Meiosis HL 28 http://sciencevideos.wordpress.com

31. Prophase II The nuclei break down. No crossing-over occurs. 10.1 Meiosis HL 29 http://sciencevideos.wordpress.com

32. 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. 10.1 Meiosis HL 30 http://sciencevideos.wordpress.com

35. Variation only in regions where crossing over has taken place in prophase I (recombination of alleles)10.1 Meiosis HL 31 http://sciencevideos.wordpress.com

36. 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. 10.1 Meiosis HL 32 http://sciencevideos.wordpress.com

37. 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. 10.1 Meiosis HL 33 http://sciencevideos.wordpress.com

38. Which phase of meiosis is shown? Why? Interphase Prophase I Metaphase I Metaphase II Reason: 10.1 Meiosis HL 34 http://sciencevideos.wordpress.com

40. Crossing-over has already taken place, so must be after prophase I.

41. Homologous pairs have not yet separated, so must be still in meiosis I (metaphase I). 10.1 Meiosis HL 35 http://sciencevideos.wordpress.com

42. Which phase of meiosis is shown? Why? Interphase Prophase I Metaphase I Metaphase II Reason: 10.1 Meiosis HL 36 http://sciencevideos.wordpress.com

44. Crossing-over has taken place.

45. Homologous pairs have not aligned at the equator. 10.1 Meiosis HL 37 http://sciencevideos.wordpress.com

46. Which phase of meiosis is shown? Why? Interphase Prophase I Metaphase I Metaphase II Reason: 10.1 Meiosis HL 38 http://sciencevideos.wordpress.com

48. Replication has taken place.

49. Crossing-over has not yet taken place. 10.1 Meiosis HL 39 http://sciencevideos.wordpress.com

50. Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis 5 marks 10.1 Meiosis HL 40 http://sciencevideos.wordpress.com

51. Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis 5 marks 10.1 Meiosis HL 41 http://sciencevideos.wordpress.com

52. Genetic Variation is almost infinite as a result of meiosis. Crossing-over in prophase I Leads to recombination of alleles on the chromosomes. Random orientation in metaphase I Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations) Random orientation in metaphase II Further genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I. 10.1 Meiosis HL 42 http://sciencevideos.wordpress.com

53. Genetic Variation is almost infinite as a result of meiosis. Crossing-over in prophase I Leads to recombination of alleles on the chromosomes. Random orientation in metaphase I Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations) Random orientation in metaphase II Further genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I. Even more variation! Random fertilisationduring sexual reproduction ensures even greater variation within the population. 10.1 Meiosis HL 43 http://sciencevideos.wordpress.com

54. Mendel’s Law of Independent Assortment “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.” A and B are different genes on different chromosomes. A is dominant over a. B is dominant over b. This only holds true for unlinked genes (genes on different chromosomes). 10.1 Meiosis HL 44 http://sciencevideos.wordpress.com

55. Random Orientation vs Independent Assortment “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.” Random Orientation refers to the behaviour of homologous pairs of chromosomes (metaphase I) or pairs of sister chromatids (metaphase II) in meiosis. Independent assortment refers to the behaviour of alleles of unlinked genesas a result of gamete production (meiosis). Due to random orientation of the chromosomes in metaphase I, the alleles of these unlinked genes have become independently assorted into the gametes. Animation from Sumanas: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html 10.1 Meiosis HL 45 http://sciencevideos.wordpress.com

56. Mendel and Meiosis “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.” Mendel deduced that characteristics were determined by the interaction between pairs of alleles long before the details of meiosis were known. Where Mendel states that pairs of alleles of a gene separate independently during gamete production, we can now attribute this to random orientation of chromosomes during metaphase I. Mendel made this deduction when working with pea plants. He investigated two separate traits (colour and shape) and performed many test crosses, recording the ratios of phenotypes produced in subsequent generations. It was rather fortunate that these two traits happened to be on separate chromosomes (unlinked genes)! Remember back then he did not know about the contents of the nucleus. Chromosomes and DNA were yet to be discovered. We will use his work as an example of dihybrid crosses in the next section. Animation from Sumanas: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html 10.1 Meiosis HL 46 http://sciencevideos.wordpress.com

57. Chromosome Collection 10.1 Meiosis HL 47 http://sciencevideos.wordpress.com

58. For more IB Biology resources: http://sciencevideos.wordpress.com 10.1 Meiosis HL 48 This is a Creative Commons presentation