NUCLEOLAR ORGANIZING REGIONS Meaning and
Definition
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Nucleolar organizing regions (NORs) are specific chromosomal regions within the nucleus of eukaryotic cells that are associated with the production and organization of ribosomes, the cellular machinery responsible for protein synthesis. These regions, composed of relatively short segments of DNA, appear as dark-staining structures within the nucleus when observed under a microscope.
NORs contain large clusters of tandemly repeated ribosomal DNA (rDNA) sequences, which encode the RNA components of ribosomes. The number and location of NORs can vary between species and even individual cells. Typically, most animal and plant cells have one or a few NORs, often found on different chromosomes.
The primary function of NORs is to facilitate the assembly of ribosomes within the nucleolus, a distinct subnuclear compartment. During the transcription of rDNA genes in the nucleolus, a large pre-ribosomal RNA molecule is synthesized and processed to form the smaller RNA components of the ribosome. Additionally, NORs play a crucial role in maintaining the integrity and stability of the genome, as changes in NOR structure or function can lead to cellular abnormalities and disease.
The observation of nucleolar organizing regions is of great value in cytogenetics and pathology, as alterations in NORs can serve as diagnostic markers for certain diseases, including cancer. Fluorescent probes targeting NORs are commonly used in molecular biology techniques, such as fluorescent in situ hybridization (FISH), to visualize and study the organization and expression of rDNA genes.
