General organisation of eukaryotic ribosomal genes. All eukaryotes show a stereotypic arrangement, with often hundreds of tandemly repeated ribosomal transcription units. Each produces a large transcript which is processed into the small subunit, the 5.8S subunit and the large subunit. The external transcribed spacer (ETS) and the internal transcribed spacers (ITS1 and ITS2) do not become part of the mature rRNA, but their sequences and structures are required for the correct processing. The dot plot comparisons above the SSU and the LSU show the conservation profiles between a chordate and an arthropod sequence. In this presentation, the sequences of two species (mouse and Drosophila) are compared and a dot is placed in the diagram at each position where 10 consecutive nucleotides match. Conserved and divergent regions become thus directly apparent. It is evident that the SSU is more conserved, interrupted by a few less conserved regions, while the LSU show larger regions of divergence.