4.4.4Additional lessons
Mitosis: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 111 of 142
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 5Recognising the Phases
In a squash of root-tip cells stained to show chromosomes, cells at different stages can be seen side by side.
Interphase
- Interphase The nucleus is intact with a visible nucleolus and the chromatin appears as a diffuse mass.
Prophase
- Prophase Chromosomes become visible as thread-like structures, the nuclear envelope is breaking down and no nucleolus is seen.
Metaphase
- Metaphase Chromosomes form a line across the middle of the cell with spindle fibres attached to them.
Anaphase
- Anaphase V-shaped chromatids are pulled towards opposite poles, and the cell begins to elongate.
Telophase
- Telophase Two groups of chromosomes at opposite poles are surrounded by re-forming nuclear envelopes.
Section 2 of 5Counting Cells
1
Choose several fields of view and count all the cells in each, recording how many are in each phase.
2
Count a large enough sample, such as 150 or more cells, to improve reliability.
3
Cells in which the chromosomes are visibly condensed are counted as being in mitosis.
4
Calculate the mitotic index from the totals.
5
Use the proportion of cells in a phase to estimate the time spent in it, assuming the duration of the cycle is known.
| Calculation | Formula |
|---|---|
| Mitotic index | cells in mitosis ÷ total cells × 100 |
| Time in phase | cells in phase ÷ total cells × cycle time |
Section 3 of 5Interpreting Results
Interpreting Results
High index
- High index A high mitotic index indicates rapid growth or repair, as in root tips, embryos and some tumours.
Low index
- Low index A low mitotic index shows that few cells are dividing, as in mature tissues.
Drawing
- Drawing Make labelled diagrams showing only visible structures, with clear lines and no shading.
Limitations
- Limitations A snapshot cannot show how long each cell has been in a phase,
- so time estimates depend on the assumption that stages are randomly distributed.
Section 4 of 5Worked Example
Question
- In a root-tip sample, 150 cells were counted and 18 were in mitosis.
- Of these, 9 were in prophase.
- Calculate the mitotic index and, for a cell cycle of 24 hours, the time spent in prophase.
- Mitotic index = 18 ÷ 150 × 100 = 12%.
- Proportion of cells in prophase = 9 ÷ 150 = 0.06.
- Time in prophase = 0.06 × 24 hours = 1.44 hours.
Answer The mitotic index is 12%, and prophase lasts about 1.4 hours (1.44 hours).
Section 5 of 5Exam Checklist & Tips
4.4.4 – Key Points for the Exam
- Mitotic index The proportion of cells in a tissue that are undergoing mitosis.
- Squash A preparation in which tissue is pressed flat to separate cells for microscopy.
- Interphase The phase between divisions when the cell grows and DNA replicates.
- Field of view The area seen through the microscope at one time.
Common Mistakes
- Counting interphase cells as being in mitosis.
- Giving the mitotic index as a fraction without multiplying by 100 when a percentage is asked for.
- Describing anaphase as when chromosomes line up at the equator.
- Basing conclusions on a very small number of cells.
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