Root-tip observations and mitotic index

What you’ll learn

• Calculate a mitotic index from a complete cell count.
• Interpret stage frequencies while recognising sampling and timing assumptions.

Understand the idea

Core practical 6 uses a root-tip preparation to observe mitosis. The actively dividing region contains cells at different cell-cycle stages. Chromosome position and appearance help identify stages: alignment at the equator supports metaphase, whereas separating chromosome groups support anaphase. Staining improves chromosome visibility and spreading cells reduces overlap, but poor preparation can still make some cells unclassifiable.

The mitotic index is the number of cells in mitosis divided by the total cells counted, including interphase. State whether you report a fraction or a percentage. Select fields using a consistent, unbiased rule and record enough cells from independent roots to represent biological variation. Choosing only the most visibly dividing region after seeing the result can bias a treatment comparison.

A snapshot's stage proportions can estimate relative stage durations only under assumptions: the population is representative and approximately asynchronous, cells are progressing through a comparable cycle, and stages are classified reliably. A treatment that arrests cells in mitosis may raise the mitotic index while slowing successful division. Therefore, a larger index does not automatically mean faster growth. Any slide preparation involving stains, acid or cutting tools follows the supervised school method.

Key formulae

$$\mathrm{mitotic\ index}=\frac{N_{\mathrm{mitotic}}}{N_{\mathrm{total}}}$$

$$\mathrm{estimated\ stage\ duration}=\frac{N_{\mathrm{stage}}}{N_{\mathrm{total}}}\times\mathrm{cell\ cycle\ duration}$$

Worked example

A representative count gives 160 interphase, 24 prophase, 8 metaphase, 4 anaphase and 4 telophase cells. Find the mitotic index. Assuming an asynchronous population with a $20\ \mathrm{h}$ cell cycle, estimate the metaphase duration.

1. The total count is $160+24+8+4+4=200$; the mitotic count is $40$.
2. The mitotic index is $40/200=0.20$, or $20\%$.
3. Metaphase represents $8/200=0.04$ of the population, giving $0.04\times20=0.80\ \mathrm{h}=48\ \mathrm{min}$ under the stated assumptions.

Answer: Mitotic index $0.20$; estimated metaphase duration $48\ \mathrm{min}$.