What the drawings show
Each field is a squash of the meristem, the dividing zone just behind the root tip, drawn the way it looks at high power. Interphase cells have one round nucleus with a nucleolus. In prophase the chromatin coils into threads inside the nucleus. In metaphase the chromosomes line up across the middle, in anaphase two groups of V-shaped chromatids pull apart, and in telophase two dense masses sit at the ends with a cell plate forming between them. Cells cut by the edge of the field are never counted.
Pick your course first. AQA writes the mitotic index as a decimal, Edexcel B papers as a percentage, and the AP Mitosis Lab compares control and treated roots with chi-square. Each sheet prints its course's rules, and the slide colors follow each course's stain: toluidine blue, acetic orcein or carbol-fuchsin.
Example: one field
Set SQB342 (AQA) shows 38 whole cells, with 27 more cut by the edge and left out. Three are in prophase, one in metaphase, two in anaphase and two in telophase, so 8 cells are in mitosis. Mitotic index = 8 ÷ 38 = 0.2105, or 0.21 to 2 decimal places. As a percentage, the way Edexcel B asks, that is 21.1%. Open set SQB342
Questions
How do you calculate the mitotic index?
Count the cells in mitosis (prophase, metaphase, anaphase and telophase) and all the whole cells in the field, then divide. 9 dividing cells out of 39 gives 9 ÷ 39 = 0.23. AQA writes it as a decimal like that; Edexcel B papers multiply by 100 and give 23.1%.
Which cells count as being in mitosis?
Cells in prophase, metaphase, anaphase or telophase: any cell where you can see chromosomes or condensed chromatin. Interphase cells count only in the total. A cell with two chromatin masses and a cell plate between them is one cell in telophase, not two.
What about cells cut by the edge of the field?
These sheets leave out every cell the edge cuts through, in the dividing count and in the total, so both numbers follow the same rule. If your teacher uses a different rule, use theirs the whole way through.
How do you work out the time spent in each stage of mitosis?
Divide the cells in that stage by the total and multiply by the length of the cell cycle. If 4 of 160 cells are in metaphase and the cycle takes 16 hours (960 minutes): 4 ÷ 160 × 960 = 24 minutes. This assumes the cells are spread evenly through the cycle.
Why is the mitotic index higher near the root tip?
Cells divide in the meristem just behind the tip. Further back they stop dividing and get longer, so the index falls toward zero. AQA's practical handbook describes meristem cells as small and square with the nucleus in the center, and the cells further back as elongated with the nucleus off-center.
How do I tell anaphase from telophase?
In anaphase the chromatids are V shapes still moving apart, with a gap between the two groups and no cell plate. In telophase they have reached the ends of the cell as two dense masses, and a cell plate is forming across the middle.
How does the AP Mitosis Lab use chi-square?
It compares interphase and mitosis counts in control and lectin-treated roots. Expected = row total × column total ÷ grand total, df = 1, and the critical value at p = 0.05 is 3.84 on the AP formula sheet. If χ² is 3.84 or more, reject the null hypothesis.
Sources, read October 27, 2026: AQA A-level Biology specification (PDF): required practical 2 and actual cell size; AQA A-level Biology practical handbook (PDF): mitotic index = cells in mitosis ÷ total cells, meristem and elongated cells; Pearson Edexcel A level Biology B, Paper 3, June 2024, question 1(c) and mark scheme: mitotic index × 100 to 1 decimal place; AP Biology Investigative Labs, Investigation 7 (PDF): lectin-treated roots, expected values and df = 1; AP Biology equations and formulas (PDF): chi-square critical values; BioNinja (IB), calculating mitotic index.