International A Level Biology / Mitosis, meiosis and reproduction
Core practical 6: the root tip squash
Edexcel International A Level Biology (YBI11), chapter 8: Mitosis, meiosis and reproduction, specification point 3.15. Free lesson.
Read the revision notes · International A Level Biology past papers
The root tip squash lets you see chromosomes and cells at different stages of mitosis under a light microscope. The method works because it separates the cells, stains the chromosomes and spreads the cells into one thin layer.
Why use the root tip?
Only the meristem, just behind the root cap, contains cells that are actively dividing by mitosis. Further back, cells in the zone of elongation get longer but have stopped dividing, so most would be in interphase. Use the end of a quickly growing root, such as garlic or onion, because it contains the meristem. The root cap protects the tip as it pushes through the soil. The meristem is the zone of cell division, so it is where you can find the stages of mitosis. The zone of elongation contains cells that grow longer but do not divide.
How to prepare the root tip squash
Each step helps you make the chromosomes visible. The acid separates cells, the stain darkens the chromosomes, and the squash spreads cells so light can pass through them.
What a successful slide shows
A good squash lets you see individual cells rather than thick clumps. The chromosomes should be dark against the cells, with some cells showing each stage of mitosis. You should see one layer of cells. The chromosomes should be stained dark. Some cells should be visible in each stage of mitosis.
Hazards and precautions
State both the hazard and the action that reduces the risk. The acid and stain can harm or stain skin, while the scalpel can cut you. Hot hydrochloric acid can damage eyes and skin. Wear eye protection and gloves. Acetic orcein is acidic and stains skin. Wear gloves and wipe up spills straight away. A scalpel has a sharp blade. Cut downwards onto a tile and keep your fingers clear.
Explaining results and writing the method
If cells appear in thick, overlapping clumps and the chromosomes are unclear, link each problem to the step that should have prevented it. A precise method names the part of the root, the stain and how the microscope is used. If the root was not in hot acid for long enough, the middle lamella may not have been hydrolysed, so cells stayed stuck together. If the squash was not pressed firmly enough, the cells may not have spread into one layer. Avoid vague instructions such as “cut a piece of root, stain it and look at it”. Say which part you cut, what you stain it with, and how you view it.