Wiktionary
n. 1 (context biology English) A small nucleus. 2 (context biology English) The smaller of the nuclei of a ciliate protozoan.
Wikipedia
The micronucleus is the smaller nucleus in ciliate protozoans, such as the Paramecium. In fission it divides by mitosis, and in conjugation it furnishes the pairing of gamete nuclei, by whose reciprocal fusion a zygote nucleus is formed, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission.
Usage examples of "micronucleus".
There are three different sets of self-duplicating nuclei, with the DNA in each set serving different purposes: a large macronucleus, governing the events in regeneration after injury, a set of eight or more micronuclei containing the parts of the genome needed for reproduction, and great numbers of tiny nuclei from which the cilia arise.