Practice question

Ribosomal DNA is organized as tandem repeats containing 18S, 5.8S, and 28S coding regions separated by internal transcribed spacers and external spacers. A probe specific to 18S sequence, under stringent hybridization conditions, anneals by complementary base pairing only to regions containing 18S coding DNA or its mature RNA transcript, not to 28S or intronic regions. This specificity allows detection of small subunit rRNA on northern blots, assessment of RNA integrity, quantification of expression, or localization of rDNA loci on Southern blots, supporting studies of phylogeny and ribosome biogenesis.