Visualization of RNA transcription and processing.

Electron microscopic visualization of transcriptionally-active chromatin dispersed by the Miller spreading technique allows a unique view of in vivo genetic events on an individual gene basis. We have used the method to ultrastructurally analyze transcription, ribonucleoprotein assembly and early RNA processing events on the pre-messenger RNA transcripts of Drosophila melanogaster Pol II genes. Our findings are surprising in two regards--splicing as a rule initials co-transcriptionally and is frequently complete before polyadenylation, and cleavage at poly(A) sites, at least for a few specific genes, occurs post-transcriptionally.