A level-readyAll subjects · Biology · WBI12 (Unit 2)
Core practical 6: root tip squash and mitotic index
Core practical 6 is a root tip squash: you stain a root tip so you can see chromosomes in cells at different stages of mitosis, then calculate the mitotic index.
Method
1. Cut about 5 mm from the tip of a growing root (for example garlic or onion). Cell division happens in the meristem just behind the tip.
2. Put it in warm hydrochloric acid (for example 1 mol dm-3 at about 60 °C) for a few minutes. The acid breaks down the middle lamella so cells separate, and stops cell division.
3. Rinse, then add a stain that binds to chromosomes, such as toluidine blue or acetic orcein.
4. Place on a slide, add a cover slip and squash gently (press through filter paper) to spread the cells into a single layer.
5. Observe at low power, then high power.
Safety: wear eye protection; hydrochloric acid is an irritant.
Mitotic index
Mitotic index = number of cells in mitosis / total number of cells counted (often x 100 to give a percentage).
Count cells in prophase, metaphase, anaphase and telophase as 'in mitosis'. A high mitotic index shows rapid cell division, so it is highest close to the root tip.
Getting good results
- Squashing to one layer of cells stops cells overlapping.
- Count cells in several fields of view and use the same region of the root each time.
- The proportion of cells in a stage is related to how long that stage lasts: the more cells in a stage, the longer it takes.
Worked example
In a field of view, 24 of 160 cells are in mitosis. Calculate the mitotic index as a percentage.
- Mitotic index = 24 / 160 x 100
Answer: 15%
Check yourself
- 36 of 240 cells are in mitosis. Calculate the mitotic index as a percentage.
Show answer
15 %. 36 / 240 x 100 = 15%.
- A student counts 12 cells in prophase, 5 in metaphase, 3 in anaphase, 4 in telophase and 176 in interphase. Calculate the mitotic index as a decimal.
Show answer
0.12. 24 in mitosis out of 200: 24 / 200 = 0.12.
- Why is the root tip placed in warm hydrochloric acid?
- To break down the middle lamella so cells separate, and stop division
- To stain the chromosomes
- To make the cells divide faster
- To dissolve the chromosomes
Show answer
To break down the middle lamella so cells separate, and stop division. Acid hydrolysis separates cells and fixes them.
- Why is the root tip squashed?
- To kill the cells
- To make chromosomes condense
- To add more stain
- To spread the cells into a single layer so they can be seen clearly
Show answer
To spread the cells into a single layer so they can be seen clearly. A single layer avoids overlapping cells.
- Why is the root tip used?
- It contains chloroplasts
- It contains a meristem where cells are dividing
- It has no cell walls
- Its cells are haploid
Show answer
It contains a meristem where cells are dividing. Meristems are regions of active mitosis.
- Which stain is suitable for showing chromosomes in a root tip squash?
- Benedict's reagent
- Iodine solution
- DCPIP
- Toluidine blue
Show answer
Toluidine blue. Toluidine blue (or acetic orcein) stains chromosomes.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.