Macronucleus

(redirected from macronuclei)
Also found in: Dictionary, Medical.
Related to macronuclei: macronuclear, Meganucleus

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Many ciliates have one or more macronuclei and micronuclei.
A repetitive DNA sequence in Paramecium macronuclei is related to the [beta] subunit of G proteins.
Finally, the nucleus splits into eight parts, four of which become macronuclei and four micronuclei.
Researchers knew that during this division, some unknown actor strips the new macronuclei of all genes related to sexual reproduction and simultaneously destroys the parental macronuclei.