Macronucleus(redirected from somatic nucleus)
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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