Calculation of Mitotic Index (Edexcel International A Level (IAL) Biology): Revision Note
Exam code: YBI11
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).
Answer:
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 = 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).
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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