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
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
- The 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).
A sample of cells from an onion root tip
Answer:
Step 1: Identify the cells undergoing mitosis
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
Exam Tip
You will need to remember the mitotic index formula as it will not be given to you.