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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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Mitosis: the stages

  • Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)

  • Although mitosis is actually one continuous process, it can be divided into four main stages

  • These stages are:

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

  • Most organisms contain many chromosomes in the nuclei of their cells (e.g. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for the sake of simplicity

  • The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

  • Chromosomes condense (and are now visible when stained)

  • The chromosomes consist of two identical chromatids called sister chromatids

    • Each sister chromatid contains one DNA molecule

    • They are joined together at the centromere

  • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)

  • Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)

  • The nuclear envelope (nuclear membrane) breaks down into small vesicles

Diagram of a prophase cell detailing spindle fibres, centromeres, chromosomes, cytoplasm, and membrane. Annotations describe nuclear and nucleolus breakdown.
Prophase

Metaphase

  • Centrosomes reach opposite poles

  • Spindle fibres (protein microtubules) continue to extend from centrosomes

  • Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles

  • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

  • Each sister chromatid is attached to a spindle fibre originating from opposite poles

Diagram of metaphase showing chromosomes aligned at the spindle equator, with centrosomes at poles and microtubules organising spindle formation.
Metaphase

Anaphase

  • The sister chromatids separate at the centromere (the centromere divides in two)

  • Spindle fibres (protein microtubules) begin to shorten

  • The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Diagram of anaphase showing chromatids moving to opposite poles, splitting at centromeres, and being pulled by microtubules with descriptive labels.
Anaphase

Telophase

  • Chromosomes arrive at opposite poles and begin to decondense

  • Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

  • The spindle fibres break down

Diagram of telophase showing two cells with re-forming nuclei, nucleolus, and cell membranes. Notes explain chromatid uncoiling and cytokinesis.
Telophase

Examiner Tips and Tricks

Make sure you learn the four stages of mitosis and what is happening to the DNA molecules (one chromatid contains one DNA molecule) at each stage.

Learn ‘PMAT’ (prophase, metaphase, anaphase, telophase) to help you remember the order of the stages!

After interphase but before the parent cell undergoes mitosis, the human parent cell nucleus actually contains 92 DNA molecules! This is because during interphase (S phase), the 46 DNA molecules in the parent cell have replicated to form sister chromatids. As human cells have a diploid number of 46 this replication results in 92 molecules.

This ensures the two daughter cells will be diploid (have 46 chromosomes each) when mitosis occurs. Remember to read the questions carefully as only human diploid cells have 46 chromosomes so if the question refers to another organism, its diploid number will be different.