High resolution mapping of in situ hybridized biotinylated DNA to surface-spread Drosophila polytene chromosomes.

We describe a method of mapping genes or transcripts on polytene chromosomes by transmission electron microscopy. We present several applications which illustrate that, in favorable cases, the method has a resolution of ca. 10 kg, and that high resolution mapping of hybridization sites relative to bands and puffs can be achieved. We mapped sites of transcription for poly-(A) RNA and present evidence which shows that these sites are localized in some bands and puffs, but are also found in interbands.