micronucleus

GO:0031040 · Membrane organelle · Membrane bounded organelle · Proposed

A membrane-bounded organelle of ciliated protozoan cells that contains a diploid copy of the cell's complete genome. Sections of contiguous sequence in the macronucleus are often interrupted by internal eliminated sequences (IES), and may be permuted, in micronuclei. Genic transcription is not found in micronuclei. Some ciliate species may contain multiple micronuclei per cell. [GO:0031040]

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
micronuclear chromosomal DNA	Dna	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Diploid germline DNA carried on micronuclear chromosomes and transmitted across sexual generations. `GO:0031040` GO:0031040 defines the ciliate micronucleus as containing a diploid copy of the complete genome. `DOI:10.7554/eLife.19090` Hamilton et al. 2016 sequenced the Tetrahymena thermophila micronuclear germline genome and assembled it into five chromosome-length super-assemblies.

Taxonomic distribution

NCBITaxon:5911 Tetrahymena thermophila — COMMON: Tetrahymena thermophila cells carry a smaller diploid germline micronucleus and a larger polyploid somatic macronucleus. [DOI:10.7554/eLife.19090]

Canonical examples

NCBITaxon:5911 Tetrahymena thermophila — Model ciliate whose strain SB210 micronuclear genome was sequenced from purified micronuclei after validating that all five micronuclear chromosomes were intact. [DOI:10.7554/eLife.19090]

Functions

germline genome maintenance — The Tetrahymena micronucleus maintains the germline genome that is transmitted across sexual generations.
- GO:0031040 GO:0031040 defines the micronucleus as containing a diploid copy of the complete genome.
- DOI:10.7554/eLife.19090 Hamilton et al. 2016 describe the Tetrahymena MIC as the transcriptionally silent germline nucleus whose genetic material is transmitted across sexual generations.

Mechanism graphs

Micronuclear DNA maintains the germline genome (FUNCTION)

In Tetrahymena thermophila, diploid micronuclear chromosomal DNA is carried in the transcriptionally silent micronucleus and preserves the genetic material transmitted through sexual generations.

Subject	Predicate	Object	Evidence
micronuclear chromosomal DNA	is housed in	micronucleus	`DOI:10.7554/eLife.19090` Hamilton et al. 2016 analyzed the Tetrahymena MIC as the diploid germline nucleus.
micronuclear chromosomal DNA	constitutes	germline genome	`DOI:10.7554/eLife.19090` Hamilton et al. 2016 sequenced the Tetrahymena MIC genome and linked most larger MIC scaffolds into five chromosome-length super-assemblies.
micronucleus	preserves	germline genome	`DOI:10.7554/eLife.19090` Hamilton et al. 2016 describe the Tetrahymena MIC as containing the genetic material transmitted across sexual generations.
germline genome	is transmitted during	sexual reproduction	`DOI:10.7554/eLife.19090` Hamilton et al. 2016 describe the Tetrahymena MIC as the germline nucleus whose genetic material is transmitted across sexual generations. `GO:0019953` GO:0019953 grounds the final graph target as the sexual reproduction process.

Discussions

CURATION_TODO (OPEN): Resolve whether a source-neutral SO identifier exactly denotes the diploid germline chromosomal DNA of ciliate micronuclei. Micronuclear chromosomal DNA is the germline DNA carried inside a ciliate micronucleus. Generic GO chromosome terms describe complete chromosomal structures rather than this ciliate-specific DNA constituent.

Evidence

GO:0031040 Gene Ontology cellular-component term for micronucleus.
GO:0005634 Gene Ontology cellular-component ancestor term for nucleus.
DOI:10.7554/eLife.19090 Hamilton et al. 2016, 'Structure of the germline genome of Tetrahymena thermophila and relationship to the massively rearranged somatic genome', eLife.
GO:0019953 Gene Ontology biological-process term for sexual reproduction.

Curation history

2026-09-22T14:24:59Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-22T14:25:45Z codex · EXPANDED_RECORD — Added nucleus parentage, reviewed-label-only micronuclear chromosomal DNA, Tetrahymena thermophila taxonomic scope and canonical example, germline genome maintenance function, DOI evidence, a nonmechanistic MIC germline genome graph, and a grounding TODO.
2026-09-22T14:28:13Z codex · REFINED_GRAPH — Changed the micronuclear chromosomal DNA to germline genome edge from encodes to constitutes.
2026-09-22T14:43:32Z codex · REVIEW_FIXES — Resolved PR #1289 review issue #1290 by replacing the ungrounded sexual_generation graph node with grounded GO:0019953 sexual reproduction and retargeting the germline-genome transmission edge.

Source: data/structures/membrane_organelle/micronucleus.yaml