Study of Mitosis in Onion Root Tip: Lab Procedure

Mitosis is the type of cell division in which one cell divides to form two identical daughter cells, and the onion root tip is the classic place to watch it happen. The tip of a growing onion root contains a region called the root apical meristem where cells divide very rapidly, so a single microscope slide can show dozens of cells caught in different stages of division.

This experiment matters because mitosis is the basis of all growth and repair in your body. By staining the root tip and observing it under high power, you can actually see chromosomes condensing, lining up and separating — the same process that heals a cut on your finger.

Key Concepts

• Mitosis: division of the nucleus into two genetically identical nuclei, followed by division of the cytoplasm.
• Stages: prophase (chromosomes condense and become visible), metaphase (chromosomes line up at the centre), anaphase (sister chromatids separate to opposite ends), telophase (two new nuclei form).
• Interphase: the resting stage between divisions when chromosomes are not distinctly visible.
• Root apical meristem: the growing tip of the root, 2 to 3 mm long, where cells are actively dividing.
• Acetocarmine: a red stain (2 percent solution) that colours chromosomes dark red so they stand out.
• Squash preparation: pressing the stained tip flat under a coverslip so cells spread into a single layer.

How It Works

1. Grow onion roots by placing an onion bulb on a water-filled bottle for 2 to 3 days until roots are 2 to 3 cm long.
2. Cut the last 2 to 3 mm of several root tips and place them on a clean slide.
3. Add a drop of acetocarmine stain, warm the slide gently over a flame for a few seconds, and let it stain for about 10 minutes.
4. Place a coverslip over the tip and press firmly (but without sliding) to squash the tissue into a thin layer.
5. Observe first under low power to find dividing cells, then under high power to identify prophase, metaphase, anaphase and telophase.

Worked Example

Suppose you scan your slide and count 60 cells that are clearly dividing: 38 in prophase, 12 in metaphase, 6 in anaphase and 4 in telophase. The percentage of cells in prophase is (38 ÷ 60) × 100 = 63.3 percent, while only (4 ÷ 60) × 100 = 6.7 percent are in telophase. This pattern is expected: prophase is the longest stage of mitosis, so at any moment most dividing cells are caught in it, while anaphase and telophase are brief and show fewer cells.

Common Misconceptions

• "Every cell in the root tip shows mitosis." Only the meristematic zone near the tip divides. Cells further up have stopped dividing and are elongating.
• "Chromosomes are visible in interphase too." In interphase the genetic material is spread out as thin chromatin and cannot be seen as distinct chromosomes.
• "You can see mitosis without any stain." Unstained chromosomes are nearly transparent; acetocarmine is what makes them visible.

Real-World Applications

• Cancer research studies mitosis because cancer is uncontrolled cell division.
• Plant breeders use root-tip studies to check chromosome numbers in new crop varieties.
• Doctors examine dividing cells in bone marrow samples to diagnose blood disorders.

To sum up, the onion root tip squash lets you see real mitosis in action: stain the growing tip, squash it flat, and identify each stage by how the dark red chromosomes are arranged. Prophase will always show the most cells because it lasts the longest, and a labelled drawing of each stage completes the practical.