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(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 It might be outdated or ideologically biased.



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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
The only stem cell line purportedly derived by human nuclear transfer has subsequently been shown to originate by parthenogenesis than by reprogramming form the somatic nucleus.
The letter C stands for "constant" or the DNA in a haploid nucleus or genome; 2C values reported in this paper, represent the DNA content of a diploid somatic nucleus. For diploid barley, the 2C complement of DNA per nucleus is 10.68 pg.
The only stem cell line purportedly derived by human nuclear transfer had subsequently been shown to originate by parthenogenesis rather than by reprogramming from the somatic nucleus. She said that as far as she had been able to ascertain, nobody had ever tried to introduce such cells into humans before.