Micronucleus

A micronucleus is the smaller nucleus in ciliate protozoans. In fission it divides by mitosis, and in conjugation 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.

Other meaning

 * In a micronucleus test, the genotoxicity of a chemical compound is tested.