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Young growing root-tip regions contain many dividing cells. In the centre’s approved preparation, suitable treatment softens tissue and staining increases chromosome contrast. A squash spreads cells into a thin layer so individual chromosomes are less obscured by overlapping tissue.
Video by Science and Plants for Schools. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Use a light microscope and calibrate a graticule
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Squashing the tissue helps by:
Reducing overlap and making a thin preparation. Preparation improves visibility.
After watching 2: Mitotic index uses as its denominator:
All cells counted. Define the population represented by the count.
What you'll be able to do
Young growing root-tip regions contain many dividing cells. In the centre’s approved preparation, suitable treatment softens tissue and staining increases chromosome contrast. A squash spreads cells into a thin layer so individual chromosomes are less obscured by overlapping tissue.
Use the specified stain, treatment and slide-handling procedure, including controls for corrosive reagents, heating and breakable glass where relevant. Pressing a coverslip vertically helps spread tissue without dragging chromosomes across the slide. These method choices affect image interpretation, not just the appearance of the preparation.
Begin with a low-power objective to locate a suitable thin region, then increase magnification and use appropriate fine focusing. Calibrate a graticule at the objective setting if measuring dimensions. Draw observed features with clear lines, sensible proportions and labels rather than inventing organelles not visible.
Classify cells by chromosome arrangement: condensed but unaligned chromosomes suggest prophase, an equatorial group metaphase, separating groups anaphase and two reforming nuclei telophase. Interphase has no corresponding set of clearly condensed mitotic chromosomes. Count representative fields with a consistent inclusion rule rather than selecting only obvious dividing cells.
Mitotic index is cells in mitosis divided by all cells counted, reported as a proportion or percentage. State whether all mitotic stages were included. A different index can reflect a changed division rate, altered stage duration or sampling differences; it is not automatically proof of faster growth.
Inferring stage duration from the fraction of cells in a stage assumes a suitable asynchronous, steady population and representative sample. A treatment that arrests cells in one stage can increase that stage’s proportion while reducing successful division. Keep observed proportions separate from the causal interpretation.
An eyepiece graticule has arbitrary divisions until compared with a stage micrometer of known scale. Align the scales, find a convenient shared distance and divide the known specimen distance by the corresponding graticule divisions. Record the objective setting; changing the objective changes how much specimen length corresponds to one graticule division.
Use low power to locate and focus a suitable region before higher power and fine focus. Magnification is image size divided by actual size with matching units; resolution describes distinguishable detail and does not improve merely by enlarging a blurred image. A scientific drawing records visible features with clear lines, proportions, labels and an appropriate scale, rather than copied textbook detail.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Question 1: After treatment, the metaphase fraction rises substantially. Why is “mitosis is faster” not the only explanation?
A stage-arrest interpretation must be considered.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Compare the eyepiece scale with a stage micrometer and record the objective setting.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.