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Biology_A-level_Cie

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  1. 1-1-the-microscope-in-cell-studies
    5 主题
  2. 1-2-cells-as-the-basic-units-of-living-organisms
    5 主题
  3. 2-1-testing-for-biological-molecules
    3 主题
  4. 2-2-carbohydrates-and-lipids
    8 主题
  5. 2-3-proteins
    6 主题
  6. 2-4-water
    2 主题
  7. 3-1-mode-of-action-of-enzymes
    5 主题
  8. 3-2-factors-that-affect-enzyme-action
    8 主题
  9. 4-1-fluid-mosaic-membranes
    4 主题
  10. 4-2-movement-into-and-out-of-cells
    12 主题
  11. 5-1-replication-and-division-of-nuclei-and-cells
    6 主题
  12. 5-2-chromosome-behaviour-in-mitosis
    2 主题
  13. 6-1-structure-of-nucleic-acids-and-replication-of-dna
    4 主题
  14. 6-2-protein-synthesis
    5 主题
  15. 7-1-structure-of-transport-tissues
    4 主题
  16. 7-2-transport-mechanisms
    7 主题
  17. 8-1-the-circulatory-system
    7 主题
  18. 8-2-transport-of-oxygen-and-carbon-dioxide
    5 主题
  19. 8-3-the-heart
    4 主题
  20. 9-1-the-gas-exchange-system
    6 主题
  21. 10-1-infectious-diseases
    3 主题
  22. 10-2-antibiotics
    3 主题
  23. 11-1-the-immune-system
    4 主题
  24. 11-2-antibodies-and-vaccination
    6 主题
  25. 12-1-energy
    5 主题
  26. 12-2-respiration
    11 主题
  27. 13-1-photosynthesis-as-an-energy-transfer-process
    8 主题
  28. 13-2-investigation-of-limiting-factors
    2 主题
  29. 14-1-homeostasis-in-mammals
    8 主题
  30. 14-2-homeostasis-in-plants
    3 主题
  31. 15-1-control-and-coordination-in-mammals
    12 主题
  32. 15-2-control-and-coordination-in-plants
    3 主题
  33. 16-1-passage-of-information-from-parents-to-offspring
    5 主题
  34. 16-2-the-roles-of-genes-in-determining-the-phenotype
    7 主题
  35. 16-3-gene-control
    3 主题
  36. 17-1-variation
    4 主题
  37. 17-2-natural-and-artificial-selection
    7 主题
  38. 17-3-evolution
    2 主题
  39. 18-1-classification
    5 主题
  40. 18-2-biodiversity
    7 主题
  41. 18-3-conservation
    6 主题
  42. 19-1-principles-of-genetic-technology
    11 主题
  43. 19-2-genetic-technology-applied-to-medicine
    4 主题
  44. 19-3-genetically-modified-organisms-in-agriculture
    2 主题
  45. 1-1-the-microscope-in-cell-studies
  46. 1-2-cells-as-the-basic-units-of-living-organisms
  47. 2-1-testing-for-biological-molecules
  48. 2-2-carbohydrates-and-lipids
  49. 2-3-proteins
  50. 2-4-water
  51. 3-1-mode-of-action-of-enzymes
  52. 3-2-factors-that-affect-enzyme-action
  53. 4-1-fluid-mosaic-membranes
  54. 4-2-movement-into-and-out-of-cells
  55. 5-1-replication-and-division-of-nuclei-and-cells
  56. 5-2-chromosome-behaviour-in-mitosis
  57. 6-1-structure-of-nucleic-acids-and-replication-of-dna
  58. 6-2-protein-synthesis
  59. 7-1-structure-of-transport-tissues
  60. 7-2-transport-mechanisms
  61. 8-1-the-circulatory-system
  62. 8-2-transport-of-oxygen-and-carbon-dioxide
  63. 8-3-the-heart
  64. 9-1-the-gas-exchange-system
  65. 10-1-infectious-diseases
  66. 10-2-antibiotics
  67. 11-1-the-immune-system
  68. 11-2-antibodies-and-vaccination
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Meiosis: sources of genetic variation

  • Having genetically different offspring can be advantageous for natural selection

  • Meiosis has several mechanisms that increase the genetic diversity of gametes produced

  • Both crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes

Crossing over

  • Crossing over is the process by which non-sister chromatids exchange alleles

  • Process:

    • During meiosis I homologous chromosomes pair up and are in very close proximity to each other

    • The non-sister chromatids can cross over and get entangled

    • These crossing points are called chiasmata

    • The entanglement places stress on the DNA molecules

    • As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

  • This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes

  • There is usually at least one, if not more, chiasmata present in each bivalent during meiosis

  • Crossing over is more likely to occur further down the chromosome away from the centromere

Diagram of chromosomes undergoing crossing over, showing bivalent formation and chiasma, indicating where breaking and rejoining occurs.
Diagram showing chromatids separating in meiosis II, illustrating parental and new combinations of alleles in blue and red chromosomes.
Crossing over of non-sister chromatids leading to the exchange of genetic material

Independent assortment

  • Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I

  • The different combinations of chromosomes in daughter cells increases genetic variation between gametes

  • In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle

    • Each pair can be arranged with either chromosome on top, this is completely random

    • The orientation of one homologous pair is independent (unaffected by the orientation of any other pair)

  • The homologous chromosomes are then separated and pulled apart to different poles

  • The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up

  • To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell

    • For humans this is 223 which calculates as 8 388 608 different combinations

Diagram showing chromosomes with two possible orientations during meiosis, highlighting independent assortment with text explanations in boxes.
Diagram illustrating meiotic division showing four possible combinations of chromosome types A, a, B, and b resulting in four types of gametes.
Independent assortment of homologous chromosomes leading to different genetic combinations in daughter cells

Examiner Tips and Tricks

Several sources of genetic variation have been outlined above. It is also worth remembering that genetic variation can occur on an even smaller scale than chromosomes: mutations can occur within genes. A random mutation that takes place during DNA replication can lead to the production of new alleles and increased genetic variation.

Fusion of gametes

  • Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment

  • This means each gamete carries substantially different alleles

  • During fertilisation any male gamete can fuse with any female gamete to form a zygote

  • This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles

  • There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Diagram showing fusion of male and female gamete nuclei forming a genetically diverse zygote, highlighting meiosis's role in diversity.
How meiosis and the random fusion of gametes affects genetic variation

Examiner Tips and Tricks

These sources of genetic variation explain why relatives can differ so much from each other. Even with the same parents, individuals can be genetically unique due to the processes outlined above.