Practical: Observing Mitosis (SQA National 5 Biology): Revision Note

Observing dividing cells from a root tip

• Cell division occurs in actively growing regions of a plant root tip

  • The root tip is where cells divide rapidly to allow the plant to grow

• Pre-prepared slides of root tips can be studied, or temporary slides can be prepared using the squash technique below

Apparatus

• Onion /garlic roots (the bulbs can be encouraged to grow roots by suspending them over water for a week or two)

• Scalpel

• Forceps

• Dilute hydrochloric acid

• Water bath (at ~60°C)

• Microscope slides and coverslips

• Pipettes

• Mounted needle

• Acetic orcein stain

• Paper towels

• Optical microscope

Method

1. Place root tips in a small beaker containing dilute hydrochloric acid

   • Warm gently in a water bath (~60°C) for about 5 minutes to soften the tissue and break down the cells

2. Remove the root tips and rinse with distilled water to remove acid

3. Cut off about 2–3 mm from the very tip of each root (the growing region) using the scalpel

4. Place the tip on a microscope slide using the forceps and add a few drops of acetic orcein stain (or other acetic acid-based stain) with a pipette

   • The acetic acid in the stain fixes the tissue and helps the chromosomes take up the dye

5. Leave the stain on the tissue for about 2 minutes

6. Gently tease apart the cells using a mounted needle

7. Carefully place a coverslip over the sample

8. Use a paper towel to press gently (or tap with the blunt end of a pencil) to spread the cells into a thin layer (this is the “squash”) — avoid sliding the coverslip

9. Observe the slide under low power, then high power, to see dividing and non-dividing cells

Expected results

• Cells undergoing mitosis (similar to those in the images below) can be seen and drawn

• Annotations can then be added to these drawings to show the different stages of mitosis

Limitations

• Sample preparation (squashing, staining, cutting) can damage cells or create artefacts — features that aren’t part of the original cell

  • Careful technique reduces artefacts but cannot eliminate them completely

• Cells and tissues are 3D, so cutting at different angles can make structures appear inconsistent in size or shape

• Optical microscopes have limited magnification and resolution, so very small structures cannot be seen clearly