4.4.8Additional lessons
Mitotic Index- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 47 of 101
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
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6What the Mitotic Index Shows
The mitotic index is a snapshot of how active division is in a tissue.
Definition
- Definition The mitotic index is the proportion of cells in a tissue sample that are undergoing mitosis at the moment of observation.
Formula
- Formula Mitotic index = number of cells in mitosis divided by total number of cells observed, usually written as a percentage.
Meaning of a high value
- Meaning of a high value A high index shows rapid division, as in root tips, embryonic tissue or fast-growing tumours.
Meaning of a low value
- Meaning of a low value A low index shows slow division, as in mature specialised tissue that rarely divides.
Section 2 of 6Applications
Cancer diagnosis
- Cancer diagnosis Pathologists compare the index of a biopsy with healthy tissue, and a high value suggests a faster-growing tumour.
Treatment monitoring
- Treatment monitoring A falling index after drug treatment suggests that the therapy is slowing cell division.
Plant growth
- Plant growth Comparing root tips under different conditions shows which factors speed or slow growth.
Comparing tissues
- Comparing tissues The index lets scientists contrast division rates between organs or between treated and untreated samples.
| Tissue | Typical mitotic index | Reason |
|---|---|---|
| Root tip meristem | High | Region of active growth |
| Mature leaf | Very low | Cells are specialised |
| Malignant tumour | Often high | Loss of cycle control |
Section 3 of 6Collecting the Data
1
Prepare a squash of the tissue, for example a stained root tip, so that the cells form a single layer.
2
Choose several fields of view at random and count every cell, noting those with visible chromosomes in prophase to telophase.
3
Add the counts together from all fields to give a large total sample.
4
Calculate the index as a percentage and repeat on further samples to get a mean.
Section 4 of 6Evaluating the Result
Sample size
- Sample size Counting only a few cells gives large random error, so aim for several hundred or more.
Phase identification
- Phase identification Misjudging early prophase or late telophase changes the count, so apply one clear rule throughout.
Snapshot only
- Snapshot only The index does not show how long division takes or how many cells will divide later.
Repeats
- Repeats Using several tissues from different organisms shows whether results are reproducible.
Section 5 of 6Worked Example
Question
- A student counts 250 cells in an onion root tip;
- 40 are in prophase, metaphase, anaphase or telophase.
- In a second sample, 300 cells contain 21 dividing cells.
- Which sample has the higher mitotic index?
- Sample 1: 40 / 250 x 100 = 16%.
- Sample 2: 21 / 300 x 100 = 7%.
- Compare16% is greater than 7%.
Answer Sample 1 has the higher mitotic index (16% against 7%), so cells in that sample are dividing more actively.
Section 6 of 6Exam Checklist & Tips
4.4.8 – Key Points for the Exam
- Mitotic index The percentage of cells in a sample that are in mitosis.
- Meristem A region of plant tissue where cells divide actively, such as a root tip.
- Biopsy A small sample of tissue removed for examination.
- Squash A preparation where tissue is flattened so cells can be viewed in a single layer.
Common Mistakes
- Counting only the cells in a single phase of mitosis instead of all four.
- Leaving interphase cells out of the total number of cells.
- Quoting an index as a number of cells rather than a percentage.
- Concluding that a high index proves cancer; healthy growing tissues also have a high index.
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