Squash an Onion and Learn the True Age of Your Cells

by carolinastaff
Onion cells undergoing mitosis

Sure, you know your birthday. You’ve huffed and puffed harder every year to blow out the candles on your special day. But do you know your physical age?

Fifteen, you say. Look at your arm; nothing you see there is 15 years old. Those skin cells are only 2 to 4 weeks old. Had a paper cut lately? The blood that oozed out of that gaping wound was probably no more than 4 months old. The cells that line your stomach bail out only 5 days after that greasy cheeseburger combo meal you devoured. Your liver cells are a little hardier; they hang around for a year and a half or so. Only a few types of cells in your body are lifetime buddies. In fact, the billion-plus cells in your body regenerate, on average, every 7 years.

Where do all those cells come from?

Does a single organ produce all those new cells, or are you born with a supply of them? Does your brain direct cell production? When your body sleeps, are your cells inactive?

No single organ produces all your cells, you’re not born with ready-made replacement cells, and your brain doesn’t direct all of the cell production going on 24/7 in your body. Cells are created 1 at a time as each cell divides.

If all those different kinds of cells are dividing, how do they know what to be when they divide? For instance, why don’t your brown eyes turn blue, or your black hair turn blond (without contacts or chemicals, respectively)? What keeps a heart from turning into an ear, or a foot from turning into an elbow? Why doesn’t a shoulder blade sprout into a wing?

What are your cells doing to preserve your unique genetic makeup?

These are the types of questions you can help your students both ask and answer. Mitosis, meiosis, cell division, and genetic inheritance are complex concepts to teach to students, and many teachers find it beneficial to approach these topics with a variety of teaching tools. Later we’ll discuss various teaching tools offered by Carolina. First, we’ll provide a hands-on activity to see mitosis up close, right under the microscope. Showing mitosis can help you begin to answer students’ questions about how their bodies function and grow.

Why use onion root tips to show all these complex changes?

Because the root tip is a fast growth area of the onion plant, cells are rapidly dividing. Also, the cells are large, so they are relatively easy to see, and the 16 chromosomes stain easily. Mitosis in Allium root tip cells is not synchronized. Students see various phases of mitosis and many intermediate configurations.

How to make an onion root tip slide and show mitosis

Follow the instructions below to prepare the onion mitosis slide by the squash method (a method most commonly used for botany specimens, but also other material that appropriately flattens so cells spread).

Onion root tip slide (squash method)



  1. Label a disposable cup “HCl” (hydrochloric acid). Pour sufficient HCl to cover the cup bottom.
  2. Label a 2nd cup “Carnoy” and pour sufficient Carnoy fluid to cover the cup bottom.
  3. Use forceps to transfer an onion root tip into the cup of HCl. Let sit for 4 minutes.
  4. After 4 minutes, transfer the root into the Carnoy fluid and let sit for 4 minutes. Remove the root from the fluid and place it on a slide.
  5. With a razor blade or scalpel, cut 1 to 2 mm off the root tip and discard all but this tip.
  6. Cover the root tip with a few drops of toluidine blue and let sit for 2 minutes.
  7. After 2 minutes, blot away the stain with a paper towel. Do not touch the root tip with your fingers or anything other than the paper towel.
  8. When blotting is complete, cover the root tip with 1 or 2 drops of water.
  9. Gently lower a coverslip over the root tip.
  10. Cover the slide with a piece of paper towel or other absorbent paper and firmly press on the coverslip (without twisting it). The pressure will spread the cells into a single layer.
  11. Observe your preparation under low microscope power. If you cannot view the cells, squash the preparation again to sufficiently separate them.
  12. Using the low power of a microscope, search the slide for cells in various stages of division. Once you locate a cell undergoing division, change to high power. Try to observe several cells in each stage. Note the important characteristics of each stage.

Carolina BioKits®: Onion Mitosis

Reagents and supplies needed to observe mitosis in onion root tips.
171130 Carolina BioKits®: Onion Mitosis

Squash Method. Prepare your own slides to observe the different phases of mitosis. This is an updated kit from our popular BioKit® series, designed for a class of 30. The Onion Mitosis BioKit® includes everything that you need to prepare onion root tips slides. Students stain the root tips, prepare squashes, and observe mitotic phases. The entire exercise can be completed in 1 lab period.

Intermediate–Easy to perform; requires some backgrou

Prepared slides

We offer hundreds of prepared slides to help you teach about microscopic specimens and processes.

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1 comment

zidane August 31, 2022 - 3:20 am

very clear and good article easy to understand. Thank you


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