Cytological bands

Polytene chromosomes

• Larval cells need a lot of RNA and protein to support rapid growth and development. These molecules are essential for building new tissues, organs, and cellular machinery quickly. Instead of dividing more often (which takes time and energy), some cells replicate their DNA without dividing to produce many copies of genes, so they can transcribe RNA and make protein much faster.
• Salivary glands are especially active because they produce large amounts of secretions, like glue proteins that help the larva attach to surfaces during pupation, and enzymes that may help with digestion of food. Some of these proteins and enzymes are secreted out into the larval environment or food, even though the larva also needs proteins and RNA for its own development. Secreting these proteins provides a functional advantage, for example, helping the larva anchor itself during pupation, aiding in digestion of food outside the body, or protecting against pathogens, which supports overall survival and successful development.
• To meet this high demand for gene expression, the DNA undergoes endoreplication, meaning it replicates multiple times without cell division. The cell repeatedly replicates its chromosomes during S-phase but skips mitosis, keeping all copies in a single nucleus.
• This repeated replication creates polytene chromosomes, which are giant chromosomes made of many aligned sister chromatids stacked side by side.
• The replicated DNA sticks together due to protein scaffolds and chromatin structure, so all the copies remain tightly aligned. This forms thick, visible chromosomes that can be seen under a light microscope, because the aligned chromatids amplify the signal of DNA staining, typically using stains like aceto-orcein, Feulgen, or DAPI.

Polytene chromosomes characteristics

• Under a microscope, the polytene chromosomes show distinct dark and light bands, which are also called cytological bands:
• Dark bands correspond to densely packed, transcriptionally inactive chromatin (heterochromatin).
• Light bands (interbands) correspond to less dense, transcriptionally active chromatin (euchromatin).
• These cytological bands provide a reproducible map of the chromosome’s structure and are used to locate genes, study mutations, and map transgenes.

Applications in fly studies:

• Scientists create Electron Micrograph (EM) maps by imaging chromosomes at high resolution. EM maps give detailed, high-resolution views of cytological bands, allowing precise cytogenetic mapping and comparison of banding patterns.
• Polytene chromosomes are used to locate genes, study mutations, and map transgenes.