Calculation of Mitotic Index (Edexcel International A Level Biology)

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Calculation of Mitotic Index

How to calculate 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 formula

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

  • You can multiply the answer by 100 if you need to give the mitotic index 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

    • 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).

Root tip mitotic index question

Root tip mitotic index solution

The cells undergoing mitosis have been identified in green

    • 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

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Alistair

Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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