Onion root tip: sampling region, mitotic index and claims about phase duration
Two candidates examine an onion root tip squash for mitosis in the HSC Biology practical. Candidate A counts 200 cells in five fields taken 1–2 mm behind the root cap and records 150 interphase, 20 prophase, 15 metaphase, 10 anaphase and 5 telophase. Candidate B counts 200 cells from a region about 10 mm behind the tip and records 190 interphase, 4 prophase, 4 metaphase, 1 anaphase and 1 telophase. Both define the mitotic index as dividing cells ÷ total cells; A obtains 25%, B obtains 5%. B concludes that mitosis is rare in onion roots and that the root tip is not the site of growth. A states that 15 of the 50 dividing cells are in metaphase, so a cell spends 30% of its cycle in metaphase, and that anaphase is shorter than prophase because fewer anaphase cells were seen. The examiner must decide which candidate's data can be used and which of the two conclusions are sound.
1. Which candidate's sample is valid, and why is the other conclusion unsound?
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Candidate A's sample is valid. The region 1–2 mm behind the root cap is the meristem, where cells are actively dividing; further back, cells are in the elongation and differentiation zones and have largely left the cell cycle. Candidate B has therefore measured the wrong tissue, not the scarcity of mitosis.
B's conclusion that the root tip is not the site of growth is contradicted by the standard observation that growth occurs just behind the tip, in the region A sampled. B's figure of 5% cannot be used as evidence about roots in general because the region was chosen wrongly and the sample is a single slide.
A's mitotic index of 50/200 = 25% is a reasonable estimate for meristematic tissue, provided the fields were chosen without bias.
2. Evaluate Candidate A's two claims about metaphase and about anaphase versus prophase.
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The metaphase claim is wrong. A divided 15 by 50, the number of dividing cells, but the fraction of the cycle spent in a phase is estimated by (cells in that phase)/(all cells scored) = 15/200 = 7.5%. The 30% figure describes the composition of the mitotic population, not the duration of the phase in the cycle. The correct statement is that about 7.5% of the cycle is spent in metaphase.
The anaphase-versus-prophase claim is sound in principle: 20 cells in prophase and 10 in anaphase out of 200 suggest roughly 10% and 5% of the cycle respectively, so anaphase is the shorter phase. This inference holds only if the cells are asynchronous and the fields were sampled at random; if cells were synchronised, or if easily recognised phases were over-scored, the proportions would not reflect duration.
3. State the limitations of this practical and how the reliability of the result could be improved.
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Limitations: one root tip and one squash; only 200 cells; fields chosen by eye, which biases towards clear or dense areas; subjective distinction between late prophase and early metaphase; overlapping cells and uneven squash thickness; uneven stain uptake making some nuclei hard to classify.
Improvements: score at least 500 cells across several roots and at least two slides; choose fields systematically, for example using a grid or by moving the stage by fixed increments; fix and stain all roots identically; have a second observer score a subsample and compare; and compare fixed distances behind the tip (1 mm, 2 mm, 5 mm, 10 mm) so that the effect of region is tested deliberately rather than accidentally.