The mitotic index is the simplest calculation in this whole chapter: count the dividing cells, count all the cells, divide one by the other. The marks are lost in the counting, not the maths — and the formula is not given to you in the exam.
📘 What you need to know
The mitotic index is the proportion of cells in a sample that are undergoing mitosis.
Mitotic index = number of cells with visible chromosomes ÷ total number of cells.
Multiply by 100 to express it as a percentage.
You must memorise the formula — it is not provided in the exam.
Root tips are used because growth in plants occurs at meristems, and the root tip meristem lies just behind the protective root cap.
Slides are prepared by the squash technique: root tips are stained then gently squashed into a thin sheet so individual cells can be seen.
Cells in interphase do not count as dividing: their chromosomes are not visible.
A high mitotic index means a high proportion of cells are dividing.
Where to look, and why
Plant growth happens in meristems, so that is where dividing cells are concentrated. The root tip meristem sits just behind the protective root cap, and contains a zone of cell division full of cells undergoing mitosis.
Pre-prepared slides can be used, or you can make a temporary slide using the squash technique: stain the root tip, then gently squash it so the cells spread into a thin layer. Squashing matters — in a thick block of tissue the cells overlap and you cannot count anything reliably.
Sample from the wrong region and your mitotic index will be far too low — older cells have stopped dividing and are elongating instead.
The formula
Learn this — it is not given in the exam
mitotic index = number of cells with visible chromosomes ÷ total number of cells
“Cells with visible chromosomes” means cells in prophase, metaphase, anaphase or telophase — all four. Cells in interphase have chromatin rather than visible chromosomes, so they go in the total but not in the top of the fraction.
🧩 How to work through a micrograph
Count the dividing cells. Go systematically across the image, marking each cell with visible chromosomes as you go so you do not double-count.
Count the total. Every cell in the field, dividing or not.
Divide the first number by the second.
Check the units asked for. A proportion (a decimal) or a percentage — and check how many decimal places the question wants.
Note the denominator: the total is 24, not 18. The dividing cells are part of the total too.
Worked examples
WORKED EXAMPLE
A straightforward count
A student counts 60 cells in a field of view, 15 of which have visible chromosomes. Calculate the mitotic index, giving your answer to 2 decimal places.
Step 1: Write the formula
mitotic index = cells with visible chromosomes ÷ total cells
Step 2: Substitute15 ÷ 60Step 3: Calculate= 0.250.25To 2 decimal places, write 0.25 — do not drop the trailing zero if the answer needs it.
WORKED EXAMPLE
Working from a table of stages
The table shows counts from an onion root tip: interphase 48, prophase 12, metaphase 6, anaphase 4, telophase 10. Calculate the mitotic index to 2 decimal places.
Step 1: Add up the dividing cells
Prophase + metaphase + anaphase + telophase
12 + 6 + 4 + 10 = 32Step 2: Find the total, including interphase48 + 32 = 80Step 3: Divide32 ÷ 80 = 0.400.40Interphase cells belong in the total but never in the top of the fraction.
WORKED EXAMPLE
Comparing two tissues
Tissue A has a mitotic index of 0.08 and tissue B has a mitotic index of 0.42. Express each as a percentage and comment on what the difference suggests.
Step 1: Convert to percentages0.08 × 100 = 8%0.42 × 100 = 42%Step 2: Compare
A much higher proportion of cells in tissue B are undergoing mitosis.
Step 3: Suggest a reason
Tissue B is dividing far more rapidly — a growth region such as a meristem, or possibly a tumour.
8% and 42%; tissue B is dividing much more rapidlyA raised mitotic index is one of the things pathologists look at in a biopsy.
💡 Exam tip
Memorise the formula. It will not be given to you.
Remember all four mitotic stages count as “dividing”, not just the obvious ones.
The total includes the dividing cells. Adding them on separately is the most common arithmetic slip.
Check whether the answer should be a decimal or a percentage, and to how many places.
If asked to sample, take cells from the meristem region just behind the root cap.
Mention the squash technique and staining if asked how the slide was prepared.
⚠ Common mix-up
Leaving interphase cells out of the total. They belong in the denominator.
Counting only metaphase cells as dividing. All four mitotic phases count.
Dividing the wrong way round. Dividing cells go on top; the index can never exceed 1.
Giving a percentage when a proportion was asked for, or vice versa.
Double-counting cells at the edge of the field. Choose a rule and apply it consistently.
Assuming a high mitotic index always means cancer. Meristems and growing embryos are high too.
That completes Cell & Nuclear Division. Try summarising the whole chapter in one line: DNA is copied in interphase, packed into chromosomes, shared out by mitosis or halved by meiosis, and the whole thing is controlled by cyclins — until a mutation stops it being controlled at all.
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