• 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

• You can multiply the answer by?100?if you need to give the mitotic index as a?percentage

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = 32 ÷ 42

mitotic index =?0.76

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

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

A sample of cells from an onion root tip

• 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

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:

actual size = size of image ÷ magnification

• The size of cells is typically measured using the?micrometre?(μm) scale, with cellular structures measured in either?micrometers?(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