makronukleus

Makronukleus, or macronucleus, is a transcriptionally active nucleus found in most ciliates that coexists with a separate germline nucleus called the micronucleus. In vegetative (non-reproductive) cells, the makronukleus governs everyday cellular functions by expressing genes needed for growth, metabolism, and maintenance, while the micronucleus preserves the genetic material used during sexual reproduction and passes it to the next generation.

Structure and genome organization vary among ciliates, but the macronucleus is typically highly polyploid, containing many copies of the genetic material. In many species the macronuclear genome is organized into numerous small chromosomes or gene-sized elements, which are present in thousands of copies. The macronucleus often divides amitotically during cell division, meaning it does not use a conventional mitotic spindle to distribute its content, while the micronucleus divides mitotically to ensure genetic continuity.

Development and lifecycle context are distinctive in ciliates. During sexual reproduction (conjugation), the existing macronucleus is degraded and a new macronucleus is formed from the micronucleus of each mating partner. This regeneration involves extensive DNA rearrangements, amplification of essential gene sequences, and elimination of nonessential or rearranged segments, producing a somatic genome optimized for vegetative function. The macronucleus is not transmitted to offspring; rather, a fresh macronucleus is generated in the new generation from the zygotic micronucleus.

Examples of ciliates with makronuclei include Paramecium and Tetrahymena. The dual-nucleus system illustrates nuclear dimorphism and the separation of germline and somatic functions in these organisms.