Which Mitotic Index Should the Class Trust?
Students prepare onion root tips, stain with acetocarmine, and count cells in one field of view to estimate the mitotic index. One pair reports 18 prophase, 4 metaphase, 2 anaphase, 1 telophase, and 75 interphase cells, for 25/100 = 25%. A second pair, working near the root tip, counts 60 cells and finds 30 in mitosis, reporting 50%. A third pair, near the basal cut end, finds no dividing cells and concludes the root tip is dead. The teacher candidate knows the meristem is a few millimeters behind the root tip and that field selection and staging criteria matter. The class must decide which estimate to trust and how to improve the protocol before writing a conclusion about the rate of mitosis.
1. Calculate each pair's mitotic index and explain why the three values differ. Which is most defensible and why?
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Calculations: Pair 1: 18 + 4 + 2 + 1 = 25 mitotic cells out of 100, so mitotic index = 25%. Pair 2: 30/60 = 50%. Pair 3: 0 divided by the number counted = 0%.
Why they differ: The values are not three estimates of the same quantity unless the sampling is equivalent. Pair 1 counted one field; that field may or may not be representative. Pair 2 worked near the root tip, where the meristem is located, and may also have chosen a dense field; 50% is high and suggests selection or a staging error. Pair 3 worked near the basal cut end, which is older tissue, so a low count is expected; zero may also reflect poor staining or scoring only interphase cells.
Most defensible: None alone. The class needs multiple random fields across the meristem and a common staging guide. Pair 1's 25% is a single-field estimate, not a class value; Pair 2's 50% is the most suspect because of field choice; Pair 3's 0% is a sampling-region problem, not evidence that the root is dead.
2. Design a sampling protocol that reduces bias and yields a class estimate with a measure of uncertainty.
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Protocol: Define stages with a projected reference image and a decision rule for ambiguous cells. Sample systematically: count 10 randomly selected fields per slide, using a grid or stage coordinates, and do not choose fields with obvious divisions. Prepare at least three slides per group and have each slide scored by two students independently. Count every cell in the field, including those with unclear borders only if the nucleus is identifiable. Record raw counts by stage. Calculate the mitotic index per field, then report the class mean, standard deviation or range, and the number of fields. This gives an estimate with uncertainty and reduces the chance that one selected field drives the conclusion. If the class mean remains around 25%, ask whether the staining or staging criteria are inflating the count.
3. A student asks whether a 25% mitotic index means mitosis takes 25% of the cell cycle. How should the teacher candidate answer?
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Answer: The mitotic index is the proportion of cells in mitosis at the moment of fixation. In an asynchronous, steady-state population, it can approximate the fraction of the cell cycle spent in mitosis, but only with caveats: all stages must be equally detectable, the population must not be synchronized, and the sample must be unbiased. A 25% index would imply roughly 25% of the cycle in M phase, which is unusually high for onion root tip and should make the class question field selection or staging. The index does not give the duration of mitosis or the length of the cell cycle. To get durations, you need time-lapse imaging or labeled mitotic figures, not a single snapshot. The teacher candidate should separate proportion of cells in mitosis from time spent in mitosis.