Calculating the mitotic index

Cell Structure, Reproduction and Development · Mitosis and the cell cycle · note 3 of 3

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Calculating the mitotic indexSpec 3.16

In short

The mitotic index is the proportion of cells in a sample that are in mitosis. It is calculated by dividing the number of cells showing visible chromosomes in a stage of mitosis by the total number of cells observed. A high mitotic index shows that many cells are dividing, as in a meristem or a rapidly growing tumour.

mitotic index = number of cells in mitosis ÷ total number of cells observed

The mitotic index can be given as a decimal, a fraction or a percentage (multiply by 100). To find it in a root tip squash, count the cells in any stage of mitosis (prophase to telophase, where chromosomes can be seen) and the total number of cells in the same field or fields of view. Cells in interphase count only in the total.

Calculating a mitotic index

In a root tip squash, a student counted 250 cells in several fields of view. 40 of these cells were in mitosis. Calculate the mitotic index as a percentage.

mitotic index = number of cells in mitosis ÷ total number of cells
mitotic index = 40 ÷ 250 = 0.16
As a percentage = 0.16 × 100

Answer: 16%

A tissue with a high mitotic index is growing or repairing itself, as in a root meristem. Comparing the mitotic index of different regions, or of tissues treated differently, shows where division is fastest. Counting a large number of cells in many fields of view makes the value more reliable.

Maths skill:

Give a percentage if the question asks for it, and check that your count of cells in mitosis is not greater than the total.

Quick check

In which phase of the cell cycle is DNA replicated?
Show answer
The S phase of interphase.
What happens to the chromosomes in metaphase?
Show answer
They line up at the equator of the cell, attached to spindle fibres by their centromeres.
Why are daughter cells from mitosis genetically identical?
Show answer
DNA is replicated exactly in interphase and each daughter cell receives one copy of every chromosome.
Why is the root tip used for studying mitosis?
Show answer
It contains meristem cells, which are dividing rapidly.
How is the mitotic index calculated?
Show answer
Number of cells in mitosis divided by the total number of cells observed.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What happens in each stage of mitosis?

In prophase the chromosomes condense and the nuclear envelope breaks down. In metaphase they line up on the equator. In anaphase the centromeres divide and sister chromatids are pulled to opposite poles. In telophase nuclear envelopes re-form and the chromosomes uncoil.

Why is mitosis important?

Mitosis produces two genetically identical daughter cells, so it is how multicellular organisms grow and replace damaged or worn-out cells. It is also the basis of asexual reproduction, in which offspring are genetic clones of one parent.

How do you make a root tip squash?

Warm a 2 to 3 mm root tip in 1 M hydrochloric acid at about 60 °C, rinse, stain with acetic orcein and squash it under a coverslip to make a single layer of cells. Then view it under a light microscope to find cells in each stage of mitosis.

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