Investigating Plant Root Tips (AQA AS Biology): Revision Note
Exam code: 7401
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 = number of cells with visible chromosomes ÷ total number of cells
The answer can be multiplied by 100 if the mitotic index is required 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).
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
Calculating the size of root tip cells
The images (micrographs) taken of slides containing root tip cells can be used to calculate the actual size of root tip cells
The magnification equation can be used:
actual size = size of image ÷ magnification
The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometres (μm) or nanometers (nm)
When doing calculations, all measurements must be in the same units
It is best to use the smallest unit of measurement shown in the question
To convert units, multiply or divide depending if the units are increasing or decreasing
Magnification does not have units
Examiner Tips and Tricks
There are 1000 nanometers (nm) in a micrometre (µm)
There are 1000 micrometres (µm) in a millimetre (mm)
There are 1000 millimetres (mm) in a metre (m)
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