macronucleus

GO:0031039 · Membrane organelle · Membrane bounded organelle · Proposed

A membrane-bounded organelle of ciliated protozoan cells that contains polyploid copies of a portion of the cell's complete genome. Transcription of genes occurs in macronuclei. Some ciliate species may contain multiple macronuclei per cell. [GO:0031039]

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
macronuclear chromosomal DNA	Dna	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Processed, amplified DNA molecules carried on ciliate macronuclear chromosomes and vegetatively expressed from the somatic genome. `GO:0031039` GO:0031039 defines the macronucleus as containing polyploid copies of part of the cell genome. `DOI:10.1371/journal.pbio.0040286` Eisen et al. 2006 sequenced the Tetrahymena thermophila macronuclear genome and described its MIC-derived, amplified chromosomes.

Taxonomic distribution

NCBITaxon:5911 Tetrahymena thermophila — COMMON: Tetrahymena thermophila cells carry a somatic macronucleus and a germline micronucleus with distinct but closely related genomes. [DOI:10.1371/journal.pbio.0040286]

Canonical examples

NCBITaxon:5911 Tetrahymena thermophila — Model ciliate whose strain SB210 macronuclear genome was shotgun-sequenced from purified macronuclei. [DOI:10.1371/journal.pbio.0040286]

Functions

somatic gene expression — The Tetrahymena macronucleus is the asexual-stage somatic nucleus in which vegetative gene expression occurs.
- GO:0031039 GO:0031039 states that transcription of genes occurs in macronuclei.
- DOI:10.1371/journal.pbio.0040286 Eisen et al. 2006 described the Tetrahymena macronucleus as the gene-expression center during asexual growth.

Mechanism graphs

Macronuclear chromosomal DNA supports somatic expression (FUNCTION)

In Tetrahymena thermophila, processed, amplified macronuclear chromosomal DNA is carried on chromosomes in the somatic macronucleus and supports vegetative gene expression.

Subject	Predicate	Object	Evidence
macronuclear chromosomal DNA	is carried on	macronucleus	`DOI:10.1371/journal.pbio.0040286` Eisen et al. 2006 described each Tetrahymena cell as carrying a MAC and MIC, and described the MAC genome as MIC-derived chromosomes that are fragmented, telomere-capped, and amplified.
macronuclear chromosomal DNA	support	somatic gene expression	`DOI:10.1371/journal.pbio.0040286` Eisen et al. 2006 state that all gene expression during asexual growth occurs in the MAC.
macronucleus	enables	somatic gene expression	`GO:0031039` GO:0031039 states that transcription of genes occurs in macronuclei.

Discussions

CURATION_TODO (OPEN): Resolve whether a source-neutral SO identifier exactly denotes the processed, amplified chromosomal DNA of ciliate macronuclei. Macronuclear chromosomal DNA is processed and amplified on the chromosomes of a ciliate somatic nucleus. Generic GO chromosome terms describe complete chromosomal structures rather than this ciliate-specific DNA constituent.

Evidence

GO:0031039 Gene Ontology cellular-component term for macronucleus.
GO:0005634 Gene Ontology cellular-component ancestor term for nucleus.
DOI:10.1371/journal.pbio.0040286 Eisen et al. 2006, 'Macronuclear Genome Sequence of the Ciliate Tetrahymena thermophila, a Model Eukaryote', PLoS Biology.

Curation history

2026-09-22T13:46:38Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-22T13:47:21Z codex · EXPANDED_RECORD — Added nucleus parentage, reviewed-label-only macronuclear chromosomes, Tetrahymena thermophila taxonomic scope and canonical example, somatic gene-expression function, DOI evidence, a nonmechanistic macronuclear chromosome graph, and a grounding TODO.
2026-09-22T13:59:15Z codex · REVIEW_FIXES — Resolved PR #1286 review issues #1287 and #1288 by renaming the macronuclear chromosome component to chromosomal DNA and tightening the housed-in graph edge evidence.

Source: data/structures/membrane_organelle/macronucleus.yaml