5.2.25 · The Mitotic Cell Cycle
Observing Mitosis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 11 of 12 Unit quiz
Lesson summary
5.2.2 Observing Mitosis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Prepare and observe a root tip squash, identify the stages of mitosis, calculate the mitotic index and make a scale drawing with magnification.
- Practical benefits
- Region used
- Interphase
- Mitotic stages
- Sample size
- Time in a stage
Part 1 of 8Why root tips are used
Meristem
- Meristem the tips of roots contain the meristem, where cells divide rapidly, so many cells are in mitosis at one time.
- (onion, garlic, broad bean)
Location
- Location the meristem lies just behind the protective root cap.
Squashing
- Squashing the cells are thin-walled and can be spread into a single layer.
Practical benefits
- Practical benefits root tips are easy to obtain and cheap, and raise no ethical issues.
Region used
- Region used cells further back are elongating or differentiating and do not divide, so only the first few millimetres are used.
Part 2 of 8Method for a root tip squash
1
Grow roots in water until a few centimetres long and cut off the terminal 2 to 3 mm with a scalpel.
2
Place the tip in warm (about 60 °C) 1 mol dm-3 hydrochloric acid for about 5 minutes,
- which breaks down the middle lamella so cells separate and stops further division (fixing).
3
Rinse in water and place the tip on a microscope slide.
4
Stain with a DNA stain such as acetic orcein, toluidine blue or Feulgen stain for a few minutes, which makes chromosomes dark.
5
Add a coverslip, cover with filter paper and press down firmly with a thumb, without twisting or sideways movement, to make a layer one cell thick.
6
Find the dividing region under low power, then use high power (×400) to identify and count cells in each stage.
- Safety
- wear eye protection, take care with acid and stains, and cut on a tile away from yourself.
Part 3 of 8Making observations
1Sampling
- Sampling choose several fields of view, count every cell in each and record the stage of each cell in a table.
2Interphase
- Interphase a cell is in interphase if it has an intact nucleus with a nucleolus.
3Mitotic stages
- Mitotic stages they are identified from the appearance of the chromosomes.
4Sample size
- Sample size count at least 100 to 200 cells so the data are reliable.
Part 4 of 8Calculating the mitotic index and stage times
1Mitotic index
Mitotic index number of cells in mitosis ÷ total number of cells counted.
- A high value shows a rapidly dividing tissue.
2Time in a stage
(cells in stage ÷ total cells) × total cell cycle time.
3Interpretation
- Interpretation the stage with the most cells is the longest.
Part 5 of 8Drawing and magnification
Drawing rules
- Drawing rules use sharp pencil lines with no shading, label with ruled lines that do not cross, draw only what is seen, show a few cells in different stages and give the magnification or a scale bar.
Magnification
Magnification size of image ÷ actual size of object.
Units
- Units measure the image in millimetres with a ruler and convert to micrometres (×1000) if the actual size is in µm.
Part 6 of 8Eyepiece graticule and stage micrometer
1
Line up the eyepiece graticule scale with the stage micrometer scale.
2
Find how many graticule divisions match a known length on the micrometer.
3
Divide the known length by the number of divisions to find the length of one division.
4
Measure a cell as a number of divisions and multiply by this length.
- Repeat for each objective
- the value of one division changes with magnification.
Part 7 of 8Evaluation
| Limitation | Improvement |
|---|---|
| Uneven squash, so cells overlap and cannot be counted | Squash more evenly to a single layer |
| Stage hard to identify, especially early prophase, and subjective | Use the same criteria for all cells, have two people count, or compare with a standard photograph |
| One slide may not be representative | Use several root tips and count more cells |
- Continuous process
- mitosis is continuous, so putting a cell in a stage involves judgement.
Part 8 of 8More from the specification
More from the specification
Genetically identical
- Having the same alleles
Spindle
- Structure of microtubules that attaches to and moves chromosomes.
Fibre
- Plant material that adds bulk and aids movement of food through the gut.
Microtubule
- Hollow protein filament used for support and transport.
Micrograph
- A photograph taken through a microscope.
Centrosome
- A pair of centrioles at right angles that organises the spindle.
Diploid
- Having two sets of chromosomes.
Centromere
- Constricted region that joins sister chromatids and binds spindle fibres.
Spindle fibres
- Protein microtubules that move chromosomes in mitosis.
Mitosis order
- Mitosis order prophase, metaphase, anaphase, telophase, remembered as PMAT.
DNA count
- DNA count after S phase the 46 human chromosomes hold 92 DNA molecules as sister chromatids, so each daughter cell receives 46.
Key terms
| Term | Meaning |
|---|---|
| Meristem | Region of plant tissue where cells divide by mitosis. |
| Mitotic index | Fraction of cells in a sample that are in mitosis. |
| Squash | Preparation in which tissue is pressed flat to give a thin layer of cells. |
| Acetic orcein | Stain that binds DNA, making chromosomes dark. |
| Magnification | Size of image divided by actual size of object. |
| Graticule | Scale in the eyepiece of a microscope. |
| Fixing | Stopping cell processes to preserve cells as they were. |
Avoid these mistakes
1
Forgetting to convert units (mm and µm) when calculating magnification.
2
Counting too few cells, so the mitotic index is unreliable.
3
Using root cells from behind the tip, which are not dividing.
4
Twisting the coverslip while squashing, which damages the chromosomes.
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