Macronucleus


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macronucleus

[¦mak·rō′nü·klē·əs]
(invertebrate zoology)
A large, densely staining nucleus of most ciliated protozoans, believed to influence nutritional activities of the cell.

Macronucleus

 

the larger (somatic) nucleus in infusorians (ciliate protozoan).

In most ciliate protozoans, the macronucleus is highly polyploid—that is, it contains several dozen to several thousand sets of chromosomes. It divides by fission and, less often, by budding, in which case full chromosome sets are distributed between the daughter nuclei. During sexual reproduction (conjugation), the macronucleus disintegrates and is replaced by a new macronucleus, which develops from the generative nucleus, or micronucleus. When this happens (and also at every division), the chromosome sets of the macronucleus multiply by endomitosis, the autonomous doubling of the number of chromosomes. The genetic apparatus of the macronucleus is active, synthesizing all types of ribonucleic acid and directing all biosynthetic processes in the cell. In the lower multinucleate infusorians, the macronuclei remain diploid and are incapable of dividing. With each division of the organism, the macronuclei are distributed between the daughter cells, resulting in a decrease in their number, and the missing macronuclei are regenerated by the micronuclei.

I. B. RAIKOV

References in periodicals archive ?
This gives an idea of functional constraints of these proteins and their adaptive evolution probably due to nuclear dualism and peculiar organization of chromatin in macronucleus of ciliates.
Entodinium kastamonicum, new species closely resembles Entodinium constrictum Dehority 1974, in shape of the body and macronucleus and the position of contractile vacuole.
During most of a cell's existence, the micronucleus is inactive, and the macronucleus directs the business of making proteins.
One of the next experiments I have planned is to look at which MDSs are connected throughout the developing macronucleus.
coli has a direct life cycle with both the trophozoite and cyst stages containing a large macronucleus and smaller micronucleus.
15 [micro]m x 5 [micro]m) has a dense polymorphic macronucleus and normally occupies an intraluminal or intraepithelial location within the digestive gland in P.
A micronucleus holds the creature's full set of genes, while a macronucleus has a restricted set that carries only essential living instructions.
Macronucleus The large kidney-shaped nucleus found in some ciliates.