| Comments |
"Establishing the number of engaged transcripts is
the first step toward explaining the behavior of
nuclear RNA (nRNA). Overall rates of RNA synthesis
in vivo can reveal the number of nascent transcripts,
providing that rates of elongation and average transcript
lengths are known.
In mammalian cells, rRNA genes give the most
reliable estimates for these values (18). In humans,
each chromosome set has ~180 copies of a 45 kbp
rRNA repeat that is clustered in tandem arrays on
chromosomes 13, 14, 15, 21, and 22. Each repeat
contains a 13 kbp transcription unit that is transcribed
and processed in nucleoli to give the 28, 18,
and 5.8S rRNAs. The repeat organization of these
highly active genes makes them easy to identify in
chromatin spreads (19), where the number of engaged
transcription complexes can be counted (see
Fig. 1)." |