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A) Head-to-tail tandem arrangement of rDNA repeat units. Typically, there are more units in an array than depicted. B) Each rDNA unit has an rRNA coding region (black) and an intergenic spacer (IGS; green). The coding region encodes the ~18S, 5.8S and ~28S rRNAs (black boxes) separated by two internal transcribed spacers (ITS-1 and 2) and flanked by two external transcribed spacers (5’- and 3’-ETS).
