The Nucleus and its Functional Domains

ID #2254

Can you explain how NOR relates to the nucleolus if at all? I am confused by it.

The nucleolus is the ribosome production facility. NOR stands for nucleolar organizing region. The NOR is the region or area that organizes the collection of 45S genes (DNA), which are used as template to make the 45S ribosomal RNA (rRNA) transcript. There are numerous repeats of the 45S gene arranged one after another on Chromosomes 13, 14, 15, 21, and 22 (in other words, the 45S gene is in tandem repeats in the NOR of certain chromosomes). During interphase, the NOR is brought to where the rRNA transcription machineries are, and this is why a nucleolus appears when an interphase cell is viewed under the microscope. In one of the slides from Lecture 22 showing the picture of the nucleolus with the different nucleolar zones (ie. fibrillar center, dense fibrillar region, granular zone), the DNA in the fibrillar center are the same as the 45S genes in tandem repeats (NOR).

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