Polytene chromosomen

Typically, when a diploid cell divides, it first duplicates each of its chromosomes. Then, the cell distributes a copy of each chromosome between the two daughter cells so that each daughter cell receives a complete set of chromosomes.

Certain cells undergo multiple cycles of chromosome duplication without undergoing any cell division. Such cells, called polyploid cells, contain multiple sets of each chromosome.

In the salivary gland cells of Drosophila larvae, this process is taken to an extreme. Here, hundreds to thousands of copies of each chromosome are generated without any cell division.

This results in the formation of unique, giant chromosomes, called polytene chromosomes.

As thousands of identical DNA sequences lay side by side, like crayons in a box, certain features in the chromatin become visible. These features exist in ordinary interphase chromosomes but are difficult to observe. 

When viewed under a microscope, an alternating pattern of dark bands and lighter interbands can be seen in the polytene chromosomes.

The bands constitute 95% of the DNA, whereas the interbands constitute 5%. 

Where the chromatin in the bands is more condensed and transcriptionally inactive, the chromatin in the interbands is less condensed and transcriptionally active. 

As Drosophila larvae progress from one developmental stage to another, specific bands and interbands greatly increase in diameter, producing structures called ‘Puffs’ or ‘Balbiani’ rings.

These puffs arise from the de-condensation of chromatin and are sites of active DNA transcription. The chromatin in puffs is arranged in looped structures, much like those observed in ‘lampbrush’ chromosomes.