# Calculation of Mitotic Index(Edexcel International A Level Biology)

## Revision Note

Author

Alistair

Expertise

Biology & Environmental Systems and Societies

## 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 = 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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