Those chromosomes which are large, multi-armed and consist of thousands of DNA are called polytene chromosomes. It is also called salivary gland chromosome. Its length is 2000 micrometers. It increases the nucleus and cell shape. Their multicopy genes result in high levels of gene expression in the organism.
Polytene chromosomes have 5 long and 1 short arm. Each arm has a black and white band. Black arms are euchromatin regions. Some parts of it swell and form large formations. It is called Balbiani ring. The chromosomal puff/ballbiani ring synthesizes mRNA. This chromosome is found in salivary gland of insects like Drosophila, Chironomus, Rhynchosciara etc. In 1881, French entomologist E.G. Balbiani discovered polytene chromosomes.