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Preparation of Chromosome Squashes

Chromosome Squashes:  
How Students Can Prepare Their Own Slides Showing the Phases of Mitosis

When you can smell the onions on my end of the hall, you know it is time for studying cell division and making slides called chromosome squashes! This remains, after 3o years of teaching, one of my favorite labs.  I think that the reason this never gets old for me, is that my students always thoroughly enjoy this lab.  They experience a range of emotions, from utter disappointment at being a slide-making failure, to giddy with delight when they peer into the microscope and realize they made "a good one."

What exactly is a chromosome squash?  A chromosome squash is the procedure of using cells from an onion root tip to show the different phases of mitosis.

How long is this lab?  This lab does take some time.  It takes a short amount of class time to start the roots growing.  After about 5 days, you will spend part of a class period cutting the roots.  And you should probably allow at least 2 days for slide making.

Are there safety precautions that should be followed?  YES!!  The lab does require the use of concentrated hydrochloric acid.  You will need a lab that is well ventilated and has a fume hood and eye wash fountain.  Students MUST wear their safety goggles and lab aprons.

Tell me how to get started!  You will need to purchase onion sets from a local feed and seed store or from a biological supply company.  Just for the sake of entertainment, I buy all three colors:  red, yellow and white.  The color of the onion does not really have any affect on the lab.  But the kids love to grow them, and they look pretty when my lab tables are full of growing onions!

The next step is to "plant" the onions.  As you can see in the photo, you will need small plastic cups and toothpicks.  A 50-mL beaker works well, also.  The student will stick three toothpicks in each onion to form a sort of tripod.  The onion is then suspended over the cup, which has been filled with water.  All of this takes about 20 minutes of one class period.  It will take around 5 days before the root tips are long enough to use.

The root tip is an area of rapid cell division.  The root grows because the cells of the root are dividing to produce more and more cells.  Once the root tips have reached a length of one inch, take a razor blade and cut the roots away from the bulb.  The roots are placed in 70% isopropyl (rubbing) alcohol.     The cells are killed and preserved by the alcohol, and since the cells are dividing, many of them will be "stopped" in one of the phases of mitosis.  Before a slide can be made, the roots must soak in the alcohol for at least 24 hours.

Day 5:  Roots are long enough to be cut.

Ready to make the slide?  You need to be forewarned that this procedure is not perfect.  Students may have to try several times before they get a slide that is decent enough to see cells clearly in the phases of mitosis.  A root tip is removed from the isopropyl alcohol and placed in a solution of hydrochloric acid and ethyl alcohol.  After a given amount of time, the root is transferred to Carnoy's solution.  Finally the slide is placed on a microscope slide and covered with aceto-orcein stain.  While in the stain, the student will chop the root tip with a razor blade.  For best results, the more chopping the better!

The last step is the "squash" part.  A cover slip is placed over the chopped onion.  Place your thumb on top of the cover slip and apply gentle pressure. This spreads the cells out over the area of the slide.

Place the slide under the microscope, and keep your fingers crossed that you got lucky.  As stated above, it is not a perfect procedure, and several attempts may have to be made in order to get a decent slide.

But....when the students peer into the microscope and realize they have a good slide, the excitement in the lab grows and is contagious.  I have my students take pictures with their cell phones.  I have never mastered the trick, but the kids are really good at it!

Have fun teaching!

Link to my Chromosome Squash Lab.

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