mikronukleus

Mikronukleus, typically rendered micronucleus in English, is a biological term describing a small nucleus or a distinct nuclear body within a cell. Its precise meaning varies by organism and cellular context. In many unicellular eukaryotes, it refers to a nucleus with specialized reproductive or genetic functions.

In ciliates such as Paramecium and Tetrahymena, the cell contains two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germline nucleus, is diploid, and participates in sexual reproduction through meiosis and fertilization. The macronucleus handles routine cell metabolism and gene expression and is derived from the micronucleus by amplification and rearrangement after conjugation.

In other contexts, micronuclei are small extranuclear bodies that form when chromosome fragments or whole chromosomes fail to be included in the daughter nuclei during cell division. The presence of micronuclei is used as an indicator of genotoxic damage in cells, and the micronucleus assay (including the cytokinesis-block micronucleus assay) is a standard test in toxicology and radiation biology for assessing chromosomal instability and DNA damage.

Origin and usage: the term derives from Latin micro- meaning small and nucleus; transliterations vary by language, with mikronukleus used in some languages such as Turkish, Russian, or Greek transliterations.

See also: micronucleus; ciliates; cytokinesis-block micronucleus assay.