Science 7–10 · Year 10

Root tip squash: seeing mitosis and counting a mitotic index

Science understanding: Biological sciences (ACARA Year 10 heredity, AC9S10U01, which names mitosis; NSW Science 7–10 (2023) Stage 5 content never uses the word mitosis, but Genetics and evolutionary change offers prepared slides of cells undergoing division as an example under the content point on defining and comparing DNA, gene, chromosome and genome, so SC5-GEV-02 is cited beside Working scientifically)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

New cells arise by mitosis in growing regions, and the fraction of cells caught dividing at any moment can be counted from a stained squash.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

1 mol/L hydrochloric acid is an irritant
Toluidine blue is harmful if swallowed and stains skin and clothing
Warm acid in the 40 °C bath
Sharp instruments

Controls

Safety glasses and gloves; rinse splashes with cold water at once
Reagents prepared by the technician; small volumes
Learners handle toluidine blue only if CSIS Appendix A or D lists it in a category that allows student use; otherwise the teacher stains the tips and adds the stain to each slide
Tongs for tubes in the bath; spills wiped at once
Clear instructions for scalpels and mounted needles

Note

Chemicals and heat: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson. Confirm the CSIS category of toluidine blue in CSIS Appendix A or D before learners use it: under CSIS Section 1 (Figure 1.7-3) a chemical not listed in Appendix A or D is treated as category N, for demonstration by approved teachers only, until an approved (Code 1) user completes a site-specific risk assessment.

What you need

Garlic cloves (the standard plant in the SAPS root tip practical; spider plant, onion and broad bean also work) suspended over water on cocktail sticks for about three days, until the roots are about 1 cm long
1 mol/L hydrochloric acid in a water bath at 40 °C, and 1 % toluidine blue stain in a dropper bottle, both prepared by the technician as the SAPS notes specify
Watch glass, slides, coverslips, mounted needle, blunt pencil for squashing, filter paper, a beaker of water for rinsing
Compound microscope at 100x and 400x, a tally counter

How to do it

Harvest roots in the morning or around midday, when the SAPS notes say active division is most likely to be seen. Cut the terminal 1 cm of several roots.
Place the root tips in 1 mol/L hydrochloric acid at 40 °C for 5 minutes, rinse in water, then stain in toluidine blue for 2 minutes, as the SAPS protocol directs.
Place a tip on a slide, keep only the terminal 2 mm (the meristem), add a drop of stain, cover, and press firmly and vertically through filter paper to spread the cells one layer thick. Make a second squash from the region 5 to 7 mm behind the tip of another treated root.
Scan at 100x for the region of small, densely packed cells, then at 400x count at least 100 cells in three separate fields of each squash.
Classify each cell as interphase (intact nucleus) or dividing (visible chromosomes in prophase, metaphase, anaphase or telophase). Mitotic index = dividing cells divided by total cells counted, expressed as a percentage.
Draw one cell in each stage seen.

What you should see

Most meristem cells are in interphase; a minority show condensed chromosomes, with metaphase plates and anaphase separations the easiest to recognise. The squash from 5 to 7 mm behind the tip shows few or no dividing cells, because in plants mitosis is restricted to meristems. The mitotic index is the learner's own count, and SAPS suggests comparing it across the class and discussing why cloves differ. The learner knows it worked when chromosomes are visible as distinct dark threads in some meristem cells and when three field counts give similar indexes.

What changes

What you change: Region of the root (terminal 2 mm versus 5 to 7 mm behind the tip)
What you measure: Mitotic index (%)
What you keep the same: Plant and harvest time
Acid and stain times and temperature
Counting rule
Cells counted per field

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Cells divide continuously; most are in interphase at any moment.
Mitosis makes gametes; it makes identical body cells.
Chromosomes are always visible in a cell.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-GEV-02 SC5-WS-01 SC5-WS-05
Australian Curriculum v9AC9S10U01AC9S10I03AC9S10I04

Sources

The pages the author read to write this activity.

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