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3.2.8Unit 3: Practical Skills in Biology I

Calculation of Mitotic Index — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Part 1 of 5What the Index Measures

The mitotic index is the proportion of cells in a sample that are undergoing mitosis at the moment of viewing.

Definition
  • Definition It is the fraction of the counted cells that have visible chromosomes, which shows they are in a stage of mitosis.
Formula
  • Formula Mitotic index = number of cells with visible chromosomes divided by the total number of cells in the field of view.
As a percentage
  • As a percentage Multiply the answer by 100 to give a percentage, if the question asks for it.
No units
  • No units It is a ratio of two counts, so a plain mitotic index has no units.

Part 2 of 5Calculating It

The same short procedure applies to every question of this type.

1
Count the cells that show visible chromosomes, which means those in prophase, metaphase, anaphase or telophase.
2
Count all cells in the sample, including cells in interphase that show only a nucleus.
3
Divide the dividing cells by the total number of cells and round to the number of decimal places asked for.
4
Check that your answer is between 0 and 1, or between 0 and 100 if it is a percentage.

Part 3 of 5Interpreting the Value

Interpreting the Value
High index
  • High index A large proportion of cells are dividing, as expected in a meristem or a rapidly growing tumour.
Low index
  • Low index Few cells are dividing, as in mature tissue that only replaces cells occasionally.
Cancer link
  • Cancer link Tissues with uncontrolled division often show an unusually high mitotic index compared with normal tissue.
Sampling
  • Sampling A larger sample from several fields of view gives a more reliable index than a count from a single field.

Part 4 of 5Worked Example

Question

  • A garlic root tip sample contains 36 cells in interphase, 14 in prophase, 5 in metaphase, 3 in anaphase and 6 in telophase.
  • Calculate the mitotic index to 2 decimal places.

  1. Cells in mitosis = 14 + 5 + 3 + 6 = 28.
  2. Total cells = 36 + 14 + 5 + 3 + 6 = 64.
  3. Mitotic index = 28 ÷ 64 = 0.4375, which rounds to 0.44.

Answer Mitotic index = 0.44 (44 % of the cells are dividing).

Part 5 of 5Exam Checklist & Tips

3.2.8 – Key Points for the Exam

  • Mitotic index The number of cells with visible chromosomes divided by the total number of cells counted.
  • Interphase cell A cell between divisions, whose chromosomes are not visible as separate structures.
  • Field of view The area of the specimen seen through the microscope at one time.
  • Sample size The number of cells counted, which affects how reliable the index is.

Common Mistakes

  • Dividing the number of dividing cells by the number of interphase cells instead of by the total number of cells.
  • Leaving telophase or prophase cells out of the count of dividing cells.
  • Quoting the answer to a different number of decimal places from the one the question requested.
  • Giving a mitotic index above 1, which is impossible because a part cannot be larger than the whole.

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