This electromicrograph shows the transcription of ribosomal RNA genes repeated in tandem in the nucleus of the amphibian Triturus viridiscens. Along each gene, many RNA polymerases are transcribing in one direction. The growing RNA transcripts appear as threads extending outward from the DNA backbone. The shorter transcripts are close to the start of transcription; the longer ones are near the end of the gene. The “Christmas tree” appearance is the result.
[Photograph from O. L. Miller, Jr., and Barbara A. Hamkalo.]