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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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Haploid & diploid cells

  • A diploid cell is a cell that contains two complete sets of chromosomes (2n)

    • These chromosomes contain the DNA necessary for protein synthesis and cell function

    • Nearly all cells in the human body are diploid with 23 pairs (46 individual) of chromosomes in their nucleus

  • Haploid cells contain one complete set of chromosomes (n)

    • In other words they have half the number of chromosomes compared to diploid cells

    • Humans have haploid cells that contain 23 chromosomes (no pairs) in their nucleus

    • These haploid cells are called gametes and they are involved in sexual reproduction

    • For humans they are the female egg and the male sperm

  • Haploidy and diploidy are terms that can be applied to cells across different species

    • They describe the number of sets of chromosomes, not the total number of chromosomes

Diagram comparing a haploid cell (n) with one set of chromosomes, and a diploid cell (2n) with two sets, using colour-coded chromosomes.
Haploid (n) and diploid (2n) cells

Examiner Tips and Tricks

Red blood cells are an exception when it comes to chromosome number as they don’t have a nucleus!

You may be asked to estimate the number of chromosomes that would be present in the haploid cell of any species. For example, dogs have 78 chromosomes in their diploid cells. When trying to find the number of chromosomes in their haploid cells simply remember that diploid is 2n and haploid is n, meaning you just need to divide the number of chromosomes by 2. So dogs have 39 chromosomes in their haploid cells!

The need for reduction division during meiosis

  • During fertilisation the nuclei of gametes fuse together to form the nucleus of the zygote

  • Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If a zygote has too many or too few chromosomes it may not survive

  • For a diploid zygote this means that the gametes must be haploid

    • n + n = 2n

    • Where n is the haploid number of chromosomes and 2n is the diploid number of chromosomes

  • Meiosis produces haploid gametes during sexual reproduction

  • The first cell division (this is not referring to the first stage) of meiosis is a reduction division

    • This is a nuclear division that reduces the chromosome number of a cell

    • In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)

  • The reduction in chromosome number during meiosis ensures the gametes formed are haploid

Diagram illustrating sexual reproduction cycle with gametes (n), zygote (2n), and adult (2n) stages, showing growth and fertilisation process.
The maintenance of chromosome number through reduction division in a mammalian life cycle