Hi,
I am confused with NADs (Nucleolus associated chromatin domains) and NORs (Nucleolus organizer regions). NOR contain multiple copies of ribosomal RNA genes (rDNA) around which nucleoli form. They are located on the short arms of the acrocentric chromosomes (13, 14, 15, 21 and 22 in humans). These regions code for 5.8S, 18S, and 28S ribosomal RNA. NADs are primarily genomic regions with heterochromatic signatures and include transposable elements (TEs), sub-telomeric regions, and mostly inactive protein-coding genes. if you see the figure 3 of this paper "Genome organization in and around the nucleolus" or in this linkhttp://www.ur.de/laengst/anemeth.html The NADs found in all chromosome but NOR found only in acrocentric chromosome. Overall nucleolus is the site of rRNA gene transcription, rRNA processing, and ribosome biogenesis. So my doubt is nucleolus forms due to NOR regions basically acrocentric chromosome then how NAD associated with all chromosome? Also, does anyone know the gtf file of chromosome position of NOR and NAD? Thanks.
Dear Ankit,
Nucleolus consists whole of the chromosomal DNA. It is more prominant due to presence of hetrochromatin. During cell replication, the DNA content of nucleolus form chromatin and subsequently chromosomes through folding and supercoiling process. And nucleolus is the site of rRNA gene transcription, rRNA processing, and ribosome biogenesis as it is the site where whole chromosomal DNA of eukaryotic cell is present. However most transcribed Premature RNA comes out of nucleolus and later forms mRNA. Hope, now you can understand how NORs and NADs get associated with chromosomes.
According to Paul Kaufman (Department of Molecular, Cell and Cancer Biology University of Massachusetts Medical School)
NADs and NORs are not the same thing. NORs are composed of the rDNA repeats, the arrays of genes encoding ribosomal RNAs that are on the 5 human p-arms. NORs form the Fibrillar Center at the heart of nucleoli, and are transcribed by a dedicated RNA Polymerase for the purpose of generating the RNA component of ribosomes.
NADs are heterochromatic regions of the genome that are frequently associated with the periphery of the nucleolus, not the center. NADs are distributed across the genome, often (but not always) overlapping heterochromatin that can also associate with the nuclear lamina. The most up to date human NAD data is here:
Article Nucleolus association of chromosomal domains is largely main...
The number and size of nucleoli can vary dependent on growth status of the cells. This likely is a function of the nucleolus being a phase-separated body
http://www.cell.com/cell/fulltext/S0092-8674(16)30492-5
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