Practical: Mitotic Index Revision Notes | A Level AQA Biology

1. Why Root Tips? The Logic Behind the Practical

Picture a growing onion root. At its very tip — within the first few millimetres — there is a zone of cells that never stop dividing. This region, called the meristematic zone, is packed with cells caught at every stage of mitosis simultaneously. That makes it the perfect tissue to study cell division under a microscope.

The AQA specification requires you to prepare stained squashes of cells from plant root tips, use an optical microscope to identify the stages of mitosis, and calculate a mitotic index. This practical also requires you to measure cell images and calculate actual cell size. Both skills appear regularly in exam questions, so understanding the method deeply — not just following steps — is what earns marks.

2. Preparing the Stained Squash

The challenge with root tip cells is that chromosomes are normally too thin and colourless to see. The preparation technique solves two problems at once: it separates the cells so they lie flat, and it stains the chromosomes so they absorb light and become visible. Here is how each stage achieves that.

Notice that the acid hydrolysis step is not about staining — it is about separating the cells. Without it, the tissue stays as a rigid block and the squash produces overlapping layers that are impossible to interpret. The stain then does the second job: aceto-orcein binds specifically to DNA, so condensed chromosomes absorb the dye and appear dark red or purple against a pale background.

3. Identifying Stages of Mitosis Under the Microscope

Once your slide is prepared, you need to recognise what you are looking at. The vast majority of cells in the root tip will be in interphase — they appear large with a clearly stained nucleus but no visible individual chromosomes. Cells in mitosis are the minority, and that ratio is exactly what the mitotic index captures.

When counting, include a cell as 'in mitosis' only if you can see distinct chromosome behaviour — condensed chromosomes, alignment, or separation. A cell with a visible nucleus but no chromosome detail is in interphase and does not count towards the mitotic numerator.

4. Calculating the Mitotic Index

The mitotic index gives you a quantitative measure of how actively a tissue is dividing. A high mitotic index means a large proportion of cells are in mitosis at any given moment — exactly what you expect in a growing root tip. In contrast, a tissue with little cell division (such as mature leaf cells) would have a mitotic index close to zero.

Worked example: Calculating the Mitotic Index from Root Tip Data

A student counts cells in a field of view of a stained root tip squash. She records 12 cells in prophase, 8 in metaphase, 5 in anaphase, 3 in telophase, and 72 cells in interphase. Calculate the mitotic index. [2 marks]

1. Cells in mitosis = 12 + 8 + 5 + 3 = 28 — Add all four mitotic stages together. Interphase cells are NOT included here — only cells actively in mitosis (prophase, metaphase, anaphase, telophase) are counted in the numerator. [M1] for correct identification of mitotic cells.
2. Total cells = 28 + 72 = 100 — Add the mitotic cells to the interphase cells to get the complete count of all cells in the field of view.
3. $$\text{Mitotic index} = \frac{28}{100} = 0.28$$ — Substitute the values into the formula. The result is a ratio. [A1] for correct final answer.
4. 0.28 means 28% of cells are actively dividing. — A mitotic index of 0.28 is high, consistent with meristematic tissue. If your answer is greater than 1, you have made an error — the numerator cannot exceed the denominator.

Answer: Mitotic index = 0.28

5. Calculating Actual Cell Size from a Microscope Image

When you look at a cell through a microscope, what you measure with a ruler or eyepiece graticule is the size of the image — not the real cell. To find the actual size of the cell, you need to account for how much the microscope has magnified it. The relationship is straightforward once you see it clearly.

The most common error in this calculation is a units mismatch. If you measure the image in millimetres but want the actual size in micrometres, you must convert before dividing. Remember: 1 mm = 1000 μm. Always state your units at every step — examiners award marks for correct units in the final answer.

Worked example: Calculating Actual Cell Size from a Micrograph

A student measures the length of a root tip cell in a micrograph and finds it is 24 mm. The total magnification of the microscope is x400. Calculate the actual length of the cell in micrometres. [3 marks]

1. $$\text{Actual size} = \frac{\text{size of image}}{\text{magnification}}$$ — [M1] for correct formula. Always write the formula first — it earns a method mark even if you make an arithmetic error later.
2. 24 mm = 24 \times 1000 = 24\,000 \, \mu\text{m} — 1 mm = 1000 μm. Converting now means the final answer will be in μm without any further adjustment. [M1] for correct unit conversion.
3. $$\text{Actual size} = \frac{24\,000}{400} = 60 \, \mu\text{m}$$ — 24000 ÷ 400 = 60. Divide the converted image size by the magnification.
4. Actual cell length = 60 μm — [A1] for correct answer with correct units. A cell of 60 μm is a reasonable size for a plant root tip cell, so this passes the sanity check.

Answer: Actual cell length = 60 μm

6. Evaluating the Practical: Reliability, Validity, and Improvements

AQA exam questions frequently ask you to evaluate this practical — not just describe it. That means identifying specific sources of error, explaining how they affect the results, and suggesting concrete improvements. Here is how to think through the main issues.

• Counting error: It can be difficult to distinguish late prophase from early metaphase, or telophase from interphase. This introduces subjectivity into the count. Improvement: have two observers count independently and compare results; use a defined counting protocol or decision tree for ambiguous stages.
• Sample size: Counting only one field of view gives an unreliable estimate. Improvement: count at least three separate fields of view and calculate a mean mitotic index.
• Acid hydrolysis time: Too short leaves cells clumped; too long destroys chromosomes. Improvement: standardise the time and temperature of the HCl treatment precisely using a calibrated water bath and timer.
• Squashing technique: Uneven pressure produces overlapping cells in some areas and gaps in others. Improvement: apply firm, even pressure with a rolling motion and check the slide before counting.
• Staining: Insufficient staining time makes chromosomes pale and hard to identify. Improvement: standardise staining time and concentration; use a positive control (a slide known to show clear chromosomes) for comparison.

7. Exam Technique: Getting Full Marks on Mitotic Index Questions

Mitotic index questions appear in two main forms: calculation questions and evaluate/explain questions. Each has a predictable mark scheme structure, and knowing that structure lets you target marks efficiently.

For calculation questions, the mark scheme almost always awards one mark for the correct formula or method, one mark for correct substitution (including unit conversion where needed), and one mark for the correct final answer with units. Write every step — if your arithmetic slips, you can still collect the method marks.

For explain questions about why a tissue has a high or low mitotic index, link the index value directly to the rate of cell division. A high mitotic index means a greater proportion of cells are dividing at any one time — this is characteristic of rapidly growing or repairing tissue. A low mitotic index indicates slow or absent cell division.