What genetic process is occurring in a puff of a polytene chromosome? how do we know this experimentally?

A giant chromosome that contains thousands of DNA strands or Puffs is known as a polytene chromosome. In insects, like Drosophila, the polytene chromosomes are most usually found in salivary glands.  

The puffs are the locations on the polytene chromosomes that are uncoiled regions and transcriptionally active. So, the genetic procedure, which is taking place in a puff of a polytene chromosome is a high level of transcription. For example, in the salivary gland of Drosophila, the polytene chromosomes go through various rounds of endoreduplication and articulate a higher concentration of a mucoprotein prior to the pupation.  

The activity of transcription can be determined with the application of precursors of radioactively labeled RNA. The high activity of transcription would exhibit a greater volume of radioactivity from the precursors of RNA.