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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Mitotic index

  • The mitotic index is the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis

  • The mitotic index can be calculated using the formula below:

mitotic index = number of cells with visible chromosomes ÷ total number of cells

  • The answer can be multiplied by 100 if the mitotic index is required as a percentage

Worked Example

A student who wanted to observe mitosis prepared a sample of cells. They counted a total of 42 cells in their sample, 32 of which had visible chromosomes.

Calculate the mitotic index for this sample of cells (give your answer to 2 decimal places).

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = 32 ÷ 42

mitotic index = 0.76

Worked Example

The table below shows the number of cells in different stages of mitosis in a sample from a garlic root tip. Calculate the mitotic index for this tissue (give your answer to 2 decimal places).

Mitotic index table, downloadable AS & A Level Biology revision notes

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = (prophase + metaphase + anaphase + telophase) ÷ total number of cells

mitotic index = (14 + 5 + 3 + 6) ÷ (36 + 14 + 5 + 3 + 6)

mitotic index = 28 ÷ 64

mitotic index = 0.44

Worked Example

The micrograph below shows a sample of cells from an onion root tip. Calculate the mitotic index for this tissue (give your answer to 2 decimal places).

Illustration of human chromosomes in various stages of cell division under a microscope, arranged in a grid pattern on a pale background.
A Sample of cells from an onion root tip

Number of cells with visible chromosomes (green) = 20

Total number of cells (green + red) = 20 + 55 = 75

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = 20 ÷ 75

mitotic index = 0.27

Rows of illustrated cells with numbers; some highlighted in green showing different mitotic stages, including metaphase and anaphase.
A sample of cells from an onion root tip

Calculating the size of root tip cells

  • The images (micrographs) taken of slides containing root tip cells can be used to calculate the actual size of root tip cells

  • The magnification equation can be used:

actual size = size of image ÷ magnification

  • The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometres (μm) or nanometers (nm)

  • When doing calculations, all measurements must be in the same units

    • It is best to use the smallest unit of measurement shown in the question

  • To convert units, multiply or divide depending if the units are increasing or decreasing

  • Magnification does not have units

Examiner Tips and Tricks

  • There are 1000 nanometers (nm) in a micrometre (µm)

  • There are 1000 micrometres (µm) in a millimetre (mm)

  • There are 1000 millimetres (mm) in a metre (m)