Nucleolus Organizer Regions (NOR) refer to specific chromosomal regions found in the nucleus of eukaryotic cells that play a crucial role in the organization and formation of nucleoli. Nucleoli are distinct subnuclear structures responsible for ribosome biogenesis - the process of creating ribosomes, the cellular machinery responsible for protein synthesis.
Typically, NORs are composed of tandem arrays of a DNA sequence called ribosomal DNA (rDNA), which codes for ribosomal RNA (rRNA). rRNA is a vital component of ribosomes.
NORs are located on one or more pairs of chromosomes known as nucleolar organizing chromosomes. In humans, these chromosomes encompass the short arms of acrocentric chromosomes (i.e., chromosomes with a centromere near one end). Acrocentric chromosomes include chromosomes 13, 14, 15, 21, and 22.
During interphase, when the cell is not actively dividing, NORs are inactive and appear as condensed regions within the nucleolus. However, as the cell prepares for cell division, NORs become active, eventually forming distinct nucleoli. This activation involves specific proteins that interact with rRNA genes and promote their transcription.
The nucleolus organizer regions are essential for the proper functioning of ribosomes. Ribosomes are critical in translating the genetic information encoded in messenger RNA (mRNA) into functional proteins. Therefore, any disruption or mutation within the NORs can affect ribosome biogenesis, leading to alterations in protein synthesis, which may have significant consequences on cellular processes and overall organism health.
