Most puffs contain perichromatin ribonucleoprotein granules 30–40 nm in diameter; other puffs contain ribonucleoprotein granules 25 nm in diameter or mixtures of these and perichromatin granules. All puffs contain fragments of band material possibly from several bands. By examining progressively smaller puffs the transcriptionally active region is shown to lie within an interband. Some transcriptionally active interbands are so small that there can be no significant contribution of decondensed band material to the interband. Up to 33% of all interbands contain significant evidence of transcription. These findings are discussed in relation to the use of the terms heterochromatin and euchromatin to describe the banding pattern of polytene chromosomes.

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