The salivary gland chromosomes in the dipteran larvae, are useful...

Polytene chromosomes are giant chromosomes found in the salivary gland cells of some dipterian larvae. They also occur in the epithelial cells of intestine, fat bodies, Malpighian tubules etc. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication/endomitosis), they become large, having a high content of DNA. Related Theory In some developmental stages, one or more bands bear conspicuous swellings called chromosome puffs or bulbs. The larger swellings are known as Balbiani rings. These puffs are made up of a core of DNA surrounded by a matrix of RNA. These regions contain actively transcribing DNA and are involved in the synthesis of different RNA types. The process of puffing is related to gene expression.