Transcription and processing of the major ribosomal RNAs in eukaryotes. The genes exist in tandem copies (several hundred repeats), separated by nontranscribed spacers. (a) The 45S transcripts are processed by removal of portions shown in tan, to yield the 18S, 5.8S, and 28S products. These are then assembled into ribosomal subunits, through addition of proteins. (b) Electron micrograph of spread nucleolar rRNA genes undergoing transcription. Tandemly arranged genes are being transcribed from top to bottom relative to the orientation of the micrograph.