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๐ฌ Think about it
Cut your finger and the skin heals in a few days. Pull out a hair and new hair grows back. How? Old and damaged cells are replaced by new ones. Cells make new cells by dividing. Let us see how.
How does our body grow and heal?
- Cells in our body can grow and divide.
- They replace old, dead or damaged cells.
- Cells grow only up to a certain size.
- Then growth happens by cells dividing into new cells.
Example:
A small cut on the skin heals as new cells replace the damaged ones.
In this Activity, we will grow onion roots, stain the root tips, and observe cells in different stages of division under a microscope.
Materials needed
A jar, water, an onion bulb, aceto-alcohol (glacial acetic acid:ethanol :: 1:3), 70 per cent ethanol, dilute HCl, aceto-carmine stain, slides, coverslips and a spirit lamp.
Procedure
1. Take a jar and fill it up with plain water.
2. Place an onion bulb over the jar so its root-bearing base is immersed in water.
3. Leave the setup for 5–6 days; cut 2–3 cm of the fresh roots and transfer them to aceto-alcohol for 24 hours, then to 70 per cent ethanol for preservation.
4. Take one or two preserved roots, wash them in water, and place them on a clean slide.
5. Put one drop of dilute Hydrochloric acid (HCl) on the root tips to soften the tissue; rinse after 10–15 minutes, then add 2–3 drops of aceto-carmine stain.
6. Leave the slide for 5–10 minutes, then gently warm it over a spirit lamp.
7. Cut the tip of the root, put a coverslip, and gently squash it with your thumb to spread the cells.
8. Observe the slide under a microscope.
2. Place an onion bulb over the jar so its root-bearing base is immersed in water.
3. Leave the setup for 5–6 days; cut 2–3 cm of the fresh roots and transfer them to aceto-alcohol for 24 hours, then to 70 per cent ethanol for preservation.
4. Take one or two preserved roots, wash them in water, and place them on a clean slide.
5. Put one drop of dilute Hydrochloric acid (HCl) on the root tips to soften the tissue; rinse after 10–15 minutes, then add 2–3 drops of aceto-carmine stain.
6. Leave the slide for 5–10 minutes, then gently warm it over a spirit lamp.
7. Cut the tip of the root, put a coverslip, and gently squash it with your thumb to spread the cells.
8. Observe the slide under a microscope.
Observation
The cells of the onion root tip are not all similar. They show different structures, matching different stages of cell division.
Explanation
The cells at a growing root tip divide continuously, a process called cell division. So different cells are caught at different stages, which is why they look different.
โ Summary
- Grow onion roots.
- Stain the root tips.
- Squash and observe.
- See division stages.
Why do onion root-tip cells look different from one another?
- The cells of a growing root tip divide continuously.
- This process is called cell division.
- So cells are caught at different stages of division.
- That is why they show different structures.
How orderly is cell division?
- Every day, about 1 per cent of our body cells are replaced.
- Both prokaryotic and eukaryotic cells divide.
- Eukaryotic cells divide in a controlled, orderly way.
- This process is called the cell cycle.
Example:
Hundreds of billions of cells in our body are replaced every single day.
