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 ?
triacum Buisson, 1923, but the shape of macronucleus and the location of micronucleus are different.
It then creates a new macronucleus that takes over for the old one.
One of the marvels of O.trifallax is the still-unknown molecular machinery that precisely excises noncoding DNA to create the new macronucleus, explains Glenn Herrick of the University of Utah in Salt Lake City.
CAVALIER-SMITH; I don't think that you should call the DNA that is eliminated in the macronucleus "junk DNA," because that designation begs the question of whether it has a non-coding function.
It might have some specific function in the micronucleus, but the DNA is transcriptionally inert in the micronucleus and absent from the macronucleus. DNA elimination is a way of amplifying and selecting only those sequences that are absolutely required to be in the macronucleus, and in that sense it's truly elegant.
Have you taken an unscrambled gene and put it in the macronucleus to determine if it is functional?
When food is plentiful, tetrahymena operates under the command of the macronucleus and splits repeatedly into identical copies of itself.
Micronucleus A small nucleus closely associated with the macronucleus in ciliates.
They had a band-like macronucleus (the big backward "C").
Macronucleus horseshoe-shaped 49.1 [+ or -] 10.3 (24-64, 28) in external diameter; 9.9 [+ or -] 2.5 (7-16, 28) thickness; distance between the terminations of macronucleus 10.5 [+ or -] 3.4 (7-17, 28).
izumovae Arthur and Lom, (1984) showed similar diameter of the adhesive disc, denticulate ring and macronucleus. The specimens of T.