Preparation of Salivary Gland Chromosomes

To prepare salivary gland chromosomes in Drosophila melanogaster.

Materials

• Stereo zoom/Dissecting microscope
• Third instar larva
• 1N HCl
• Physiological saline (0.7% NaCl)
• Lacto aceto orcein
• 45% acetic acid
• Nail polish or wax for sealing
• Slides and cover slips

Procedure

1. Dissect the salivary gland of the third instar in physiological saline.
2. Place it in 1N HCl for 2–3 min.
3. Transfer it to 2% Lacto Aceto orcein stain for 30 min.
4. Squash it with freshly prepared 45% acetic acid.
5. Seal the edges of cover slips with nail polish or wax.
6. Observe under the microscope for polytene chromosome.

Description

1. Edouard-Gérard Balbiani, in 1881, observed salivary gland chromosomes in Chironomous larva.
2. Theophilus Painter discovered the same in Drosophila melanogaster.
3. The polytene chromosomes are the largest chromosomes available for cytological studies.
4. These chromosomes are clearly seen in the third instar larva of Drosophila melanogaster.
5. The salivary gland chromosomes undergo somatic pairing and endoduplication without separation.
6. This multistranded chromosome contains 1024 chromosomal fibrils.
7. When stained, chromonema shows bands and interbands.
8. Along the length, there are bulged regions called Balbiani rings, or puffs,
   which are the sites of genetic action.
9. Thus, this chromosome has a common chromocenter, with 5 long areas
   radiating outwards.