FIGURE 10-40 General structure of eukaryotic pre-rRNA transcription units. The three coding regions (red) encode the 18S, 5.8S, and 28S rRNAs found in ribosomes of higher eukaryotes, or their equivalents in other species. The order of these coding regions in the genome is always 5′→3′. Variations in the lengths of the transcribed spacer regions (blue) account for most of the difference in the lengths of pre-rRNA transcription units among different organisms.