Bands & condensed chromatin as sites of transcription in polytene chromosomes of Drosophila

The question of involvement of DNA of bands and other condensed chromatin masses in transcription In polytene chromosomes of salivary glands of Drosophila larvae has been analysed by electron microscopy and autoradiography (at light and electron microscope level) following a 5 min in vitro pulse. labelling with 3H-uridine. Ultrastructural studies show that many of the thick bands have regions of lesser electron density within their 3-dimensional structure. Typical perichromatin granules are seen to be specifically associated with the less electron dense regions (perichromatin) of several thick and thin bands and also with perichromatin around the condensed masses in beta beterochromatin of chromocentre. Autoradiographic studies show that certain bands and the condensed masses of beta heterochromatin incorporate 3H-uridine to a significant level. Majority of the interbands are not labelled with 3H-uridine. It is suggested that in polytene chromosomes, bands and other condensed chromatin masses (like the beta beterochromatin) may become transriptionally active without apparent decondensation. Most of the interbands probably never transcribe