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. Author manuscript; available in PMC: 2023 Apr 26.
Published in final edited form as: Trends Genet. 2022 Jun;38(6):587–597. doi: 10.1016/j.tig.2022.02.005

Figure 2. Anatomy of a ribosomal DNA (rDNA) repeat unit.

Figure 2.

Example of a 45S rDNA repeat unit from an organism with S-type rDNA. The repeat units are represented in a tandem orientation. Each repeat unit contains a transcription unit, consisting of the 18S, 5.8S, and 28S rRNAs that are cotranscribed with two external transcribed spacers (ETS1 and ETS2) and two internal transcribed spacers (ITS1 and ITS2) that are removed post-transcriptionally. Each repeat unit also has an intergenic spacer (IGS) that is not part of the rRNA transcription unit, and may contain regulatory sequences, enhancers of the rRNA promoter, pseudogenes, origins of replication, and small repeat sequences such as Alu and short interspersed nuclear element (SINE) elements, depending on the organism.