4.4.4Additional lessons
Mitosis- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 43 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 6Recognising Each Stage
Stained root tip squashes and animal slides show cells caught at different stages.
Interphase
- Interphase The nucleus is intact with a visible nucleolus and the chromatin looks granular, because chromosomes are not yet condensed.
Prophase
- Prophase Chromosomes appear as distinct threads or rods scattered through the cell, with no clear nuclear outline.
Metaphase
- Metaphase Dark chromosomes form a neat line across the middle of the cell, and the spindle may show faintly.
Anaphase
- Anaphase Two groups of chromatids are separating, often V-shaped because the centromeres lead as they are pulled towards the poles.
Telophase
- Telophase Two clusters of chromosomes sit at opposite ends, with a constriction or new wall forming between them.
Section 2 of 6Drawing and Labelling
Draw only what is seen
- Draw only what is seen Use clear single lines, label the chromosomes and the cell outline, and do not add structures that are not visible.
Scale
- Scale Add a scale bar or state the magnification so that sizes can be calculated.
Label lines
- Label lines Draw straight label lines that do not cross, ending exactly on the structure named.
Cell choice
- Cell choice Pick a typical cell in a clear stage instead of one that is overlapping or damaged.
Section 3 of 6Estimating Time in Each Phase
The more cells found in a phase, the longer that phase is likely to last.
1
Examine several fields of view and classify every dividing cell as prophase, metaphase, anaphase or telophase.
2
Count the total number of cells in the sample, including those in interphase.
3
Calculate the proportion of cells in each phase as a percentage of the total.
4
Multiply that proportion by the total length of the cell cycle to estimate how long the phase lasts.
Section 4 of 6Reliability
Sample size
- Sample size Counting more cells reduces the effect of chance, so use several fields and record at least a few hundred cells.
Random fields
- Random fields Choose fields without bias, rather than only picking areas rich in dividing cells.
Ambiguous cells
- Ambiguous cells Set a rule for borderline cells before counting and apply it consistently to every field.
Section 5 of 6Worked Example
Question
- In a root tip sample, 200 cells were counted: 150 in interphase, 20 in prophase, 12 in metaphase, 6 in anaphase and 12 in telophase.
- The cell cycle lasts 20 hours.
- Estimate the duration of prophase.
- Check the total150 + 20 + 12 + 6 + 12 = 200.
- Proportion in prophase = 20 / 200 = 0.10, or 10%.
- Duration = 0.10 x 20 hours.
Answer
- Prophase lasts about 2 hours;
- by the same method metaphase is 1.2 hours, anaphase 0.6 hours and telophase 1.2 hours.
Section 6 of 6Exam Checklist & Tips
4.4.4 – Key Points for the Exam
- Micrograph A photograph taken through a microscope.
- Field of view The area of the specimen visible at one time through the microscope.
- Magnification The ratio of image size to actual size.
- Squash A preparation where tissue is pressed flat so cells form a single layer for viewing.
Common Mistakes
- Drawing shaded areas or sketch lines instead of clear outlines.
- Mistaking a cell in early anaphase for metaphase because the chromatids have only just moved apart.
- Counting only dividing cells and leaving out interphase cells when calculating percentages.
- Estimating phase length from one field of view.
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