Transmission electron micrograph of transcription of tandemly arranged ribosomal RNA genes. Amphibian oocyte nuclei contain numerous extra-chromosomal nucleoli, each of which contains a tandem array of genes encoding ribosomal RNA. Here, using the methods developed by O.L. Miller Jr. and colleagues, the extrachromosomal nucleoli from a newt oocyte have been dispersed in low salt, and centrifuged through a sucrose cushion containing formaldehyde on to a carbon coated grid and stained for TEM imaging. Active transcription is seen by the tree-like structures in which each branch represents a growing transcript. Untranscribed spacer DNA is seen between each transcript.