noun, plural: micronuclei
The ciliates are a group of protozoans that are known for having cilia, which are hair-like organelles, as well as the presence of two types of nuclei: the macronucleus and the micronucleus.1 A macronucleus is the larger nucleus whereas the micronucleus is the smaller nucleus. Apart from their relative sizes, the micronucleus and the macronucleus differ based on their genetic features. The micronucleus is a diploid nucleus whereas the macronucleus is a polypoloid nucleus. They also differ in terms of function: the micronucleus is involved in reproductive functions whereas the macronucleus is associated with non-reproductive functions, such as cell metabolism and protein synthesis. The large size of the macronucleus is due to its multiple sets of chromosomes. The macronuceus is said to have originated from the micronucleus through the process of DNA polytenisation. In this process, the genome of the micronucleus is amplified resulting in many copies of a single set of chromosomes. This occurs usually during conjugation.
Word origin: micro- (small) + nucleus
1 Cavalier-Smith, T. (2000). Flagellate megaevolution: the basis for eukaryote diversification. In: Leadbeater, B.S.C., Green, J.C. (eds.). The Flagellates. Unity, diversity and evolution. London: Taylor and Francis, pp. 361-390, p. 362.