Nucleolus
Say it: noo-KLEE-uh-lus
The nucleolus is a dense region inside the nucleus, with no membrane of its own, where ribosomal RNA is made and built into ribosome subunits.
Also: nucleoli
Definition
What is the nucleolus?
The nucleolus is the most prominent structure inside the nucleus of most eukaryotic cells, seen under the light microscope as a dark-staining spot. It is not enclosed by a membrane. Instead, it forms around the parts of certain chromosomes that carry many repeated copies of the genes for ribosomal RNA (rRNA), and a cell may have one nucleolus or several.
Its job is to make ribosomes. Inside the nucleolus, the enzyme RNA polymerase I transcribes the rRNA genes, the long transcripts are trimmed and chemically modified, and they are combined with ribosomal proteins imported from the cytoplasm. The result is the large and small ribosomal subunits, which leave through the nuclear pores and come together in the cytoplasm only when they meet a messenger RNA to translate. Since every protein in the cell is made on ribosomes, nucleoli tend to be large in cells that produce a lot of protein.
The nucleolus disperses as a cell enters mitosis and reforms in each new nucleus during telophase. Pathologists looking at tissue samples treat enlarged, conspicuous nucleoli as a sign of fast-growing cells, including many cancer cells.
Word parts
Where does the word “nucleolus” come from?
- nucle-
- kernel, nutLatin nucleus
- -olus
- small (diminutive)Latin nucleolus, little kernel
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Terms that go with it
- NucleusThe nucleus is the membrane-bound organelle that holds a eukaryotic cell's chromosomes and controls which of its genes are used and when.
- RibosomeA ribosome is a molecular machine of RNA and protein that reads messenger RNA and joins amino acids in the order it specifies to build a protein.
- RNARNA is a usually single-stranded nucleic acid, built with the sugar ribose and the base uracil, that carries and uses genetic information to make proteins.
- TranscriptionTranscription is the process in which a cell copies the DNA sequence of a gene into a matching strand of RNA.
- ChromatinChromatin is the complex of DNA and proteins, mainly histones, that packs a eukaryotic cell's genome into its nucleus and makes up its chromosomes.
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Written for biofideX in plain words, after checking these references:
- NHGRI Talking Glossary: Nucleolus
- OpenStax Biology 2e, 4.3 Eukaryotic Cells
- NCBI MeSH: Cell Nucleolus
- NCI Dictionary of Cancer Terms: nucleolus
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