Onion Root Tip: Reading a Mitosis Count
A student examines a prepared slide of an onion root tip under a microscope and counts 200 cells in the zone of active division, classifying each by nuclear appearance. Results: interphase 168; prophase 12; metaphase 8; anaphase 6; telophase 6. A classmate says: 'Mitosis is clearly rare in roots - only 32 cells out of 200 were dividing, so most root cells never divide.' The teacher notes the whole mitotic cycle (interphase + mitosis) takes about 20 hours in onion root at this temperature.
1. Calculate the percentage of cells in each mitotic phase and in interphase, and use the counts to estimate the duration of mitosis.
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Calculation:
- Interphase: 168/200 = 84 %
- Prophase: 12/200 = 6 %
- Metaphase: 8/200 = 4 %
- Anaphase: 6/200 = 3 %
- Telophase: 6/200 = 3 %
- Total mitosis = 32/200 = 16 %
Duration estimate: on a random snapshot, the share of cells in a stage equals the share of cycle time spent there.
- Mitosis: 0.16 × 20 h ≈ 3.2 h
- Interphase: 0.84 × 20 h ≈ 16.8 h
Conclusion: mitosis is short relative to interphase, but it is not rare - it happens in every dividing cell, just briefly.
2. Evaluate the classmate's statement. What is the logical error?
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Verdict: the statement is wrong.
The error is confusing the proportion of cells caught in a stage with the frequency of the process. A cell spends most of its cycle in interphase (growth and DNA replication, ~84 % of the time), so at any instant most cells are expected to look like interphase. The 16 % in mitosis is exactly what a 3.2 h mitotic phase in a 20 h cycle predicts.
Supporting evidence: the root tip grows continuously, so new cells are produced constantly; the zone of division is defined by the presence of mitotic figures. The counts show a normal, actively dividing meristem, not a tissue that fails to divide.
A valid criticism of the method would be: 200 cells is a modest sample, and phase assignment is subjective - but that affects precision, not the conclusion.
3. Predict how the counts would change if the root tip were kept at a much lower temperature, and explain the biological reason.
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Prediction: the total share of mitotic cells would fall and interphase would rise, because the cycle lengthens; at the same time the relative order of stages (prophase > metaphase > anaphase ≈ telophase) would be broadly preserved.
Reason: mitosis depends on enzymes (DNA polymerase, spindle proteins, separase) whose activity is temperature-dependent. Lower temperature slows these reactions, so each stage takes longer - but if the whole cycle is slowed proportionally, the snapshot proportions stay similar. The clear change appears when cooling blocks the cycle at a checkpoint (e.g. spindle formation fails), producing an accumulation of cells in one stage.
Key point: proportions reflect relative durations, so the interpretation must always state the assumption that all cells cycle at the same rate.