Figure 1.—

Organization of the ribosomal RNA (rRNA) genes in eukaryotes. The genes are organized into tandemly repeated units as diagrammed at the top. A typical unit is shown in expanded detail. The positions of the three rRNA genes (18S, 5.8S, 28S) are indicated with solid boxes, while regions processed from the primary transcript are in open boxes (ETS, external transcribed spacer; ITS, internal transcribed spacer). Between the transcription units are the intergenic spacers (IGS), which in most species are composed of one or more internally repeated sequences (shaded arrowheads). The extent and direction of the transcribed region of each unit as well as the final mature rRNAs derived from that transcript are shown at the bottom as dotted arrows.