Why use a root tip?
Plant growth occurs in regions called meristems. A root-tip meristem contains a zone where many cells are undergoing division, making it suitable for studying mitosis.
The meristem lies just behind the protective root cap. Onion or garlic roots can therefore provide tissue containing cells at different stages of the cell cycle.
A temporary slide can be prepared using the squash technique. Staining makes chromosomes easier to distinguish, while squashing spreads the cells into a thin layer so that individual cells can be viewed.
Preparing the squash
- Use a young onion or garlic root tip; approximately 1 cm of the tip can be removed.
- Place the root-tip tissue in ethanoic alcohol to fix it, preventing mitosis from continuing.
- Treat the tissue with warm dilute hydrochloric acid at about 60 °C to help separate the cells.
- Transfer the root tip onto a microscope slide and add a suitable stain such as acetic orcein, which makes the chromosomes appear strongly coloured.
- Add a coverslip and press down firmly using a paper towel. This spreads the cells into a layer thin enough for individual cells to be distinguished.
- Observe the prepared squash using an optical microscope.
Equipment used for the preparation can include a scalpel, forceps, pipettes, slides, coverslips, a water bath, stain, paper towels and an optical microscope.
Exam Tip: Be able to explain the purpose of the preparation steps rather than only listing them: fixing stops further change, hydrochloric acid helps separate cells, staining makes chromosomes visible and squashing produces a thin layer of cells.