NORs

NORs may refer to more than one concept in science and technology, notably nucleolar organizer regions in genetics and NOR gates in digital electronics.

In genetics, a nucleolar organizer region (NOR) is a chromosomal segment that contains tandem repeats of ribosomal RNA genes. NORs direct the formation of the nucleolus and the transcription of ribosomal RNA by RNA polymerase I. In humans and many primates, active NORs are associated with the acrocentric chromosome arms and can be visualized by silver staining (Ag-NOR) to indicate transcriptionally active rDNA. The number, size, and activity of NORs vary among individuals and species, and they can shift during development or in response to cellular stress. NORs are used in cytogenetics as markers for chromosome identification, to study chromosomal rearrangements, and to investigate genome organization and evolution. Epigenetic factors, including DNA methylation and histone modifications, influence which rDNA repeats are active, contributing to variation in nucleolus formation between cells.

In electronics, NOR refers to the NOT-OR logic gate. A NOR gate outputs a true (high) signal only when all inputs are false; equivalently, it computes the negation of the OR operation. A NOR gate is functionally complete, meaning any Boolean function can be constructed using only NOR gates. NOR logic is foundational in the design of simple digital circuits and has historical use in transistor-transistor logic (TTL) and complementary metal-oxide-semiconductor (CMOS) technologies. Through combinations of NOR gates, one can implement basic operations such as NOT, OR, and AND, enabling the construction of complex combinational and sequential circuits.