Transcribing rRNA genes first seen in Miller spreads of the amplified rRNA of extrachromosomal nucleoli from salamander oocytes. Each rRNA transcription unit (rTU) was about 2.5 micrometers long, corresponding to a molecule of 8,000 base pairs. The rRNA molecules attached to the chromatin fiber give rTUs a Christmas tree morphology (hence the term Miller tree), and such rTUs are arranged in tandem, separated by fiber-free spacers. Each nascent transcript ends distally with a terminal ball. This represents a processing complex that forms when the 5′- external transcribed spacer (ETS) region of the rRNA interacts with a group of specific polypeptides. See Chronology, 1969, Miller and Beatty; 1976, Chooi; preribosomal RNA, ribosomal RNA genes, 5.8S rRNA.
Subjects: Genetics and Genomics.