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Polytene Chromosomes
from Salivary Glands

1994 Woodrow Wilson Collection


Student Handout

Background

The salivary gland cells in the larval stages of Drosophila contain large, multistranded polytene chromosomes. Polytene chromosomes are produced by repeated replication during synapsis without separation into daughter nuclei. This laboratory procedure outlines the mechanism by which salivary glands are removed and prepared so that the polytene chromosomes may be observed. Drosophila virilis is used instead of Drosophila melanogaster because D. virilis is much larger and it is easier to dissect and remove the salivary glands from the larvae of this species.

Materials

For each student group:

binocular stereomicroscope
compound microscope
2 teasing needles
insect pins
scalpel
microscope slides
cover glasses
stock supply of Drosophila virilis larvae
dropping bottle of aceto-orcein stain
dropping bottle of 45% acetic acid solution

Procedures

Removing the salivary glands:

  1. Remove a large larva from the stock of D. virilis. Larger larvae are easier to dissect. However, select an active larva and one that has not started to pupate.

  2. Using the stereomicroscope, dissect the larva by placing one teasing needle on the posterior aspect of the larva and the other needle at the anterior end, near the black mouth parts. (Diagram 1)

    Drosophila larva: Internal Structures

  3. Carefully pull outward with the anterior needle. (Diagram 2)

    Procedure for removing Drosophila salivary glands

  4. There are two transparent salivary glands located anteriorly in the larva. The glands are characterized by a granular, bead-like appearance. A narrow, white ribbon of fat surrounds the glands and should be torn away.

  5. Discard all of the larva except for the salivary glands.

Staining and Observing:

  1. Place 2 drops of aceto-orcein stain on the salivary glands, and let it stand for 10 minutes.

  2. Place a cover slip over the glands, and using your thumb and a paper towel, push down on the slide. The pressure applied will squash the glands, rupture the nuclear membrane, and free the chromosomes.

  3. Using a compound microscope, observe the slide under low and high magnification.

  4. Make the slide permanent by brushing along the edges of the cover slip with clear nail polish.

Questions to answer while observing the slide:

  1. Can you detect the banding pattern? ______________________________

  2. Can you see a nucleolus? _______________________________________

  3. Can you determine the chromosome number for D. virilis from the slide preparation you have made? _______________________________________

  4. Would you expect the salivary gland cells to possess the monoploid or the diploid number of chromosomes for this species? Why or why not? _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________

  5. What is meant by synapsis?______________________________________

  6. In what kinds of cells do you ordinarily expect synapsis to occur? _____________________________________________________________

  7. Why is the question of synapsis raised in connection with the salivary gland chromosomes of Drosophila? _____________________________________________________________

  8. Draw the polytene chromosomes as you see them under the microscope.

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