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First teaching 2023

First exams 2025

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Mitotic Index: Skills (HL) (HL IB Biology)

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Marlene

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Marlene

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Determining Mitotic Index

Investigating mitosis in root tissue

  • Growth in plants occurs in specific regions called meristems
  • The root tip meristem can be used to study mitosis
  • The root tip meristem can be found just behind the protective root cap
  • In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis
  • Pre-prepared slides of root tips can be studied or temporary slides can be prepared using the squash technique (root tips are stained and then gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)

Micrograph showing a stained root tip, downloadable AS & A Level Biology revision notes

Micrograph showing a stained root tip

Analysis

  • Cells undergoing mitosis (similar to those in the images below) can be seen and drawn
  • Annotations can then be added to these drawings to show the different stages of mitosis
  • TheĀ mitotic index can be calculated

The 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:

  • 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 = 0.76

Worked example

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

Stages of cell cycle Number of cells
Interphase 36
Prophase 14
Metaphase 5
Anaphase 3
Telophase 6

Answer:

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, downloadable AS & A Level Biology revision notes

A sample of cells from an onion root tip

Answer:

Step 1: Identify the cells undergoing mitosis

Root tip mitotic index solution, downloadable AS & A Level Biology revision notes

Number of cells with visible chromosomes (green) = 20

Step 2: Count the total number of cells

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

Step 3: Substitute numbers into the equation

Mitotic index = 0.27

Examiner Tip

You will need to remember the mitotic index formula as it will not be given to you.

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.