This should be practiced several times before trying to demonstrate with students.

This activity used Drosophila melanogaster which you can obtain from any biological supply company. These organisms are frequently used in research since they are easily cultured in the lab, are prolific breeders,have a short generation time and have only four pairs of chromosomes which are easily seen under a microscope.

This illustration shows the three pair of autosomes (II,III and IV) and one air of sex chromosomes.

Drosophila melanogaster chromosome squash.

Materials:

Drosophlia melanogaster in the third instar larvae stage (largest ones)

dissecting microscope

oil immersion microscopes

2 dissecting probes (both bent and straight)

slides and cover slips

saline solution

acetocarmine stain

Optional:

video microscopy equipment and monitor

Procedure:

1. Place the larvae under the dissection microscope with the mouth side up. Note: The mouth side will have two black spots.
2. Add a drop of saline solution.
3. Hold the lower part of the body with the bent probe while inserting the straight probe into the larvae at the mouth part.
4. Pull the probe out and you should have the salivary gland trailing the probe. It should look like two elongated transparent structures.
5. Transfer the gland to a clean slide.
6. Apply a drop of acetocarmine stain and allow to dry for five minutes.
7. Place a cover slip over the contents and firmly press the gland.
8. Place the slide under the microscope and locate the chromosomes. They will appear as dark spotted areas on the slide.
9. Switch to a higher magnification and use oil the view the chromosomes. you should be able to see all four chromosomes and some banding.
10. If you have video microscope capabilities, display the slide on the television screen and have students complete the following questions. If you do not have video microscope capabilities prepare several slides and allow students to work in groups to answer the questions at individual microscopes.

Here is a sample of what you should see. You can see the four chromosomes and some banding.