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Polytene chromosomes are giant chromosomes found in certain cells, such as the salivary glands of Drosophila (fruit flies). They are formed by multiple rounds of DNA replication without separation of the replicated chromatids, resulting in many copies of the chromosome lying side by side. Polytene chromosomes are characterized by their banding pattern that results from condensed and non-condensed chromosomal regions.

A puff, also known as a Balbiani ring, is a region in a polytene chromosome that appears swollen or expanded. These puffs are the sites where the chromatin has decondensed, allowing for active transcription of genes. The genetic process occurring in a puff is the transcription of DNA into RNA. Transcription is the first step of gene expression, where the genetic information in a DNA sequence is copied into a complementary RNA sequence.

There are several experimental techniques that have been used to demonstrate that transcription is occurring in puffs of polytene chromosomes. Two important experiments are: 1. Autoradiography: Researchers can incubate the cells containing polytene chromosomes with radioactively-labeled nucleotides, which are the building blocks of RNA. These labeled nucleotides will be incorporated into newly synthesized RNA molecules during transcription. Afterward, the cells are fixed and the chromosomes visualized using autoradiography. The presence of radioactive signals in the puffs indicates that new RNA molecules are being synthesized, which confirms that transcription is occurring in these regions. 2. In situ hybridization: This technique involves the use of a labeled RNA or DNA probe that is complementary to a specific RNA sequence to be detected. By applying this probe to the cells containing polytene chromosomes, it can be determined if the RNA transcripts are present and localize them to specific chromosomal puffs. This confirms that transcription is occurring in the puffs. These experimental methods provide evidence that puffs in polytene chromosomes are sites of active transcription.