Edexcel A Level (IAL) Biology -3.16 Calculation of Mitotic Index- Study Notes- New Syllabus

Mitotic Index

🌱 Introduction

The mitotic index (MI) measures the proportion of cells undergoing mitosis in a tissue at a given time.
Useful to study cell division, growth rates, or effects of chemicals/radiation on cells.

Definition

Mitotic Index (MI) = fraction or percentage of cells in mitosis.

Formula:
$$ \text{Mitotic Index (MI)} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells observed}} \times 100 $$

📌 Only cells in prophase, metaphase, anaphase, or telophase are counted as “in mitosis.” Interphase cells are excluded.

How to Calculate

1. Prepare a squash or slide (e.g., root tip squash).
2. Observe under a microscope (×40/×100).
3. Count total number of cells in the field of view.
4. Count number of cells in mitosis (prophase, metaphase, anaphase, telophase).
5. Apply formula:
   $$ \text{MI (\%)} = \frac{\text{Cells in mitosis}}{\text{Total cells}} \times 100 $$

Example Calculation

Total cells observed = 200
Cells in mitosis = 50
$$ \text{MI (\%)} = \frac{50}{200} \times 100 = 25\% $$ ✅ Interpretation: 25% of cells are actively dividing.

Significance of Mitotic Index

⚡ Quick Recap
MI = % of cells in mitosis.
Formula: $$\text{MI (\%)} = \frac{\text{Cells in mitosis}}{\text{Total cells}} \times 100$$
Count only prophase, metaphase, anaphase, telophase.
Higher MI → rapid division; lower MI → slow division.
Used for growth analysis, cytotoxicity, and cancer research.