Nucleolus

Definition

A prominent, membrane-less subcompartment within the eukaryotic nucleus where ribosomal RNA (rRNA) is transcribed, processed, and assembled with proteins to form ribosomal subunits. In arthropods, nucleolus size and number vary with cell metabolic activity; highly active secretory cells in silk glands or venom glands typically display enlarged nucleoli, while condensed chromatin in some insect polytene chromosomes reveals distinct nucleolar organizing regions.

Etymology

Latin, diminutive of nucleus ('kernel, inner part')

Example

In the polytene salivary gland chromosomes of Drosophila melanogaster, the nucleolus appears as a distinct bulbous structure where the ribosomal DNA repeats form the nucleolar organizing region on the X chromosome, visible under light microscopy as a site of intense RNA synthesis.

Related Terms

• Nucleus
• Ribosome
• rRNA
• nucleolar organizing region
• polytene chromosome
• chromatin
• euchromatin
• heterochromatin
• Transcription
• cell nucleus

Usage Notes

Distinguish from 'nucleus' (the entire organelle) and 'nucleoid' (the analogous but unstructured region in prokaryotes). Nucleolus number varies: most arthropod cells contain one, but multiple nucleoli may form when nucleolar organizing regions are present on multiple chromosomes or when nucleolar fusion is incomplete. The term is not used for prokaryotes.