Study of Polytene Chromosomes from Chironomus Larva

 

Practical No. 1

Study of Polytene Chromosomes from Chironomus Larva

Aim

To prepare and study polytene chromosomes from the salivary glands of Chironomus larva.

Requirements

1. Live Chironomus larvae
2. Compound microscope
3. Glass slides and cover slips
4. Fine needles and forceps
5. Watch glass
6. Blotting paper
7. Aceto-carmine or aceto-orcein stain
8. Dissecting microscope
9. Dropper

Principle

Polytene chromosomes are giant chromosomes formed due to repeated replication of chromosomal DNA without cell division (endomitosis). They occur in the salivary gland cells of dipteran larvae such as Chironomus. These chromosomes show characteristic dark and light bands useful for cytological studies.

Procedure

1. Take a healthy Chironomus larva in a watch glass containing water.
2. Transfer the larva onto a clean slide containing a drop of physiological saline or stain.
3. Hold the anterior end with forceps and gently pull the posterior end using a needle.
4. Salivary glands appear as transparent elongated structures.
5. Remove the salivary glands carefully and place them in a drop of aceto-carmine/aceto-orcein stain.
6. Leave for 5–10 minutes for proper staining.
7. Place a cover slip gently over the glands.
8. Press softly with blotting paper to squash the tissue evenly.
9. Observe under low power and then high power of the microscope.

Observation

Large banded polytene chromosomes are observed in the salivary gland cells. Alternating dark bands and light interbands are clearly visible. The chromosomes are much larger than ordinary chromosomes.

Result

Polytene chromosomes from the salivary gland cells of Chironomus larva were successfully prepared and studied.

Precautions

1. Use fresh larvae for better chromosome visibility.
2. Avoid excessive pressure while squashing.
3. Do not allow air bubbles under the cover slip.
4. Use proper staining for clear banding patterns.