Mitotic Index Virtual Lab: Root Tip Squash and Chi-Squared

Q: Can the class link open on Compare samples?

Yes. On the Share page, pin Screen to "Compare samples" in the link's starting values. You can also pin Root treatment and Root region sampled. Otherwise step 3 tells students to open the tab.

Q: Should students use Auto-count at all?

Yes, after they have counted one field by hand and used Check. The hand count teaches the stages; Auto-count gives enough cells for the statistics.

Updated 2026-10-07

This mitotic index virtual lab follows the root tip squash practical of A-level, AP and IB Biology. Students hydrolyse an onion root tip in hydrochloric acid, stain and squash it, then classify cells at ×400 as interphase, prophase, metaphase, anaphase or telophase. They compare the mitotic index of root regions and lectin-treated roots with a chi-squared test and estimate how long each stage lasts. No hot acid or toxic stains are needed.

Root-tip squash practical – stages of mitosis and mitotic index

Curriculum links

AQA A-level Biology 7402, required practical 2 (section 3.2.2): stained squashes of root tip cells, identifying the stages of mitosis and calculating a mitotic index.
AP Biology Unit 4 (cell cycle, topic 4.6) and the AP cell division investigation, which compares the mitotic index of control and lectin-treated roots with a chi-squared test and counts Sordaria asci.
IB Biology D2.1 (cell and nuclear division): identifying the phases of mitosis in micrographs and calculating a mitotic index.

Simulic is not affiliated with or endorsed by AQA, the College Board or the IB.

Before the lab (5 min)

Ask: "Which part of an onion root has the higher mitotic index: the region 0–2 mm from the tip or the region 5–10 mm from the tip?" On a class link this is question 1; the simulation unlocks after students answer.

Method in the simulation

On Prepare the slide, keep Hydrolysis in HCl at 5 min and Stain at Toluidine blue, and press Play steps. Try 0 and 20 min too.
On Microscope, choose a stage button, tap every whole cell in the field and press Check. Count one field yourself before you use Count for me.
On Compare samples, keep Root treatment on Control (water) and Region on 0–2 mm from tip. Press Auto-count 300 cells three times (samples S1–S3).
Set Region to 5–10 mm from tip and press Auto-count 300 cells once (S4). Choose Sample A S1 and Sample B S4 and read the chi-squared panel.
Set Region back to 0–2 mm, Root treatment to Lectin, and auto-count three more samples (S5–S7). Compare S1 with S5.

Sample	Treatment	Region (mm)	n
S1–S3	Control	0–2	300 each
S4	Control	5–10	300
S5–S7	Lectin	0–2	300 each

the Compare samples screen of a freshly opened simulation after pressing Auto-count 300 cells three times with Control (water) and 0–2 mm from tip, then once with Region 5–10 mm from tip: bars for S1–S4 labelled 14.0, 14.0, 17.0 and 1.3 %, and the chi-squared panel for S1 and S4 showing χ² = 34.00

Expected results

Fields are drawn at random, so groups get different counts. The model's control index is 14 % at 0–2 mm, 5 % at 2–5 mm and 0.8 % at 5–10 mm; lectin raises it to about 24 %. The ranges come from 100,000 runs of the model, checked in the browser.

One 300-cell control sample at 0–2 mm: almost always between 8 % and 21 %. The mean of three samples falls between 10 % and 18 %.
One browser run: S1–S3 gave 14.0, 14.0 and 17.0 % (mean 15.0 %); S4 at 5–10 mm gave 1.3 %, with χ² = 34.00, far above 3.84.
Lectin: S5–S7 gave 20.0, 24.3 and 22.0 %. Yet S1 against S5 gave χ² = 3.83: not significant. In about 1 run in 8, a single pair of 300-cell samples misses significance. Pooling the three samples of each (135 of 900 against 198 of 900 cells in mitosis) gives χ² ≈ 14.6.
Stage durations: prophase is about half of the mitotic cells. With a 24 h cell cycle, the pooled 900 control cells give prophase lasting 1.0–2.4 h (2.0 h in the browser run).

Questions for students

(Prediction, asked again after the lab) Which region has the higher mitotic index: 0–2 mm or 5–10 mm from the tip?
When you compare control and lectin-treated roots, which variables must stay the same?
Auto-count three 300-cell control samples at 0–2 mm. What is their mean mitotic index?
Use the prophase counts of those three samples and a 24 h cell cycle. How long does prophase last?
A student compares one control and one lectin sample and gets χ² = 3.83. What should they conclude, and how could they improve the investigation?

Answers for teachers: (1) 0–2 mm, the meristem; 5–10 mm is the elongation zone. (2) The region sampled, the hydrolysis time and the stain. (3) Accept 10–18 %. (4) Prophase cells ÷ 900 × 24 h; accept 1.0–2.4 h. (5) χ² is below 3.84, so the null hypothesis is not rejected: the difference could be chance. Count more cells, from more roots, and pool the samples before testing again.

Common misconceptions

"Interphase is a resting stage." Cells grow and copy their DNA in interphase, the longest stage.
"The mitotic index counts only metaphase cells." It counts every cell in prophase, metaphase, anaphase or telophase.
"A bigger percentage difference proves an effect." S1 and S5 differ by 6 percentage points and still fail the test.
"Colchicine speeds up mitosis." It stops the spindle forming, so cells pile up in metaphase.

Extension

Colchicine: auto-count treated roots and explain the many metaphase cells.
Sordaria (AP): on the Sordaria tab, classify asci as 4:4 or crossover. Ten fields with Count for me gave 101 crossover asci out of 199: 50.8 %, so 25.4 map units.

FAQ

Can the class link open on Compare samples?

Yes. On the Share page, pin Screen to "Compare samples" in the link's starting values. You can also pin Root treatment and Root region sampled. Otherwise step 3 tells students to open the tab.

Why are my counts different from my neighbour's?

Each field is drawn at random, as on a real slide. That is why the questions accept a range.

Should students use Auto-count at all?

Yes, after they have counted one field by hand and used Check. The hand count teaches the stages; Auto-count gives enough cells for the statistics.