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

BiologyCell BiologyAges 15–16

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A virtual practical: hydrolyse onion root tips in hydrochloric acid, stain and squash them, then view the cells at ×400. Tap each cell to classify it as interphase, prophase, metaphase, anaphase or telophase, count several fields to find the mitotic index and the percentage of cells in each stage, and estimate how long each stage lasts. Compare control roots with lectin- or colchicine-treated roots and regions at different distances from the tip using a chi-squared test, and count Sordaria asci to find a crossover frequency and a gene–centromere map distance.

Lesson: Mitosis – root-tip squash and mitotic index

What it shows

Cells divide by mitosis mainly in the meristem just behind the root cap. In a root-tip squash, hot hydrochloric acid breaks down the middle lamella so the cells separate, and a DNA stain makes the chromosomes visible. Because cells divide out of step, the share of cells in each stage reflects how long that stage lasts. The mitotic index, cells in mitosis divided by all cells, compares how fast tissues divide; a chi-squared test shows whether a treatment really changes it. Sordaria asci show crossing over between a gene and its centromere.

How to use

In Prepare the slide, set Hydrolysis in HCl and Stain, then use Next step or Play steps. In Microscope, pick a stage button, tap whole cells, press Check, then Next field; press Record sample when you have over 200 cells. In Compare samples, press Auto-count 300 cells for each treatment and region, then choose Sample A and Sample B. In Sordaria, classify the mature asci.

Parameters you can change

  • Screen Prepare the slide, Microscope: count cells, Compare samples, Sordaria: crossing over
  • Root treatment Control (water), Lectin, Colchicine
  • Root region sampled 0–2 mm from tip, 2–5 mm from tip, 5–10 mm from tip
  • Hydrolysis time in 1 M HCl at 60 °C 0 min (skipped), 5 min, 20 min
  • Stain Toluidine blue, Acetic orcein, No stain
  • Cell cycle length 8–40 h
  • Show answers

Questions to explore

  1. Why is the mitotic index much higher 0–2 mm from the tip than 5–10 mm from the tip?
  2. Why are prophase cells usually more common than anaphase cells, and how does this let you estimate stage durations?
  3. Why does colchicine raise the mitotic index even though it stops cells completing mitosis?

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