The Nucleus and its Functional Domains

ID #2420

What exactly is the NOR made of? There are many NORs in the nucleolus, right? Since there are 5 chromosomes with tandem arrays(of the 5.8S, 28S, and 18S genes) on them, are there 5 NORs per nucleolus or can a chromosome have multiple NORs.

A nucleolar organizing region (NOR) is the name for the tandem arrays of 45S rDNA that codes for 45S pre-rRNA. Each chromosome (13, 14, 15, 21 and 22) has one NOR. An NOR on a given chromosome starts with the promoter for the first 45S rRNA gene, and ends with the terminator for the last (approximately 25th) 45S rRNA gene. There are approximately 25 copies of the 45S rRNA gene on each chromosome. There are actually 10 NORs, not five, because there is a maternal and paternal copy of each chromosome (e.g. 2 copies of 13, 2 copies of 14, etc.). 
Keep in mind that there are not 5.8S, 28S, or 18S genes. There is only a 45S gene that encodes the 45S pre-RNA transcript, and then the transcript is processed (cleaved) into the mature 28S, 18S and 5.8S rRNAs. 

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