A the micronucleus is the smaller nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 in ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protozoans, such as the paramecium. In fission it divides by mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

, and in conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

 furnishes the pairing of gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

 nuclei, by whose reciprocal fusion a zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

 nucleus is formed, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission.

Other meaning

  • Micronucleus is also the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. In newly formed red blood cells in humans, these are known as Howell-Jolly bodies. In normal people and many other mammals, which do not have nuclei in their red blood cells, the micronuclei are removed rapidly by the spleen. Hence high frequencies of micronuclei in human peripheral blood indicate a ruptured or absent spleen. In mice, these are not removed, which is the basis for the in vivo Micronucleus test.

In a micronucleus test
Micronucleus test
A micronucleus test is a test used in toxicological screening for potential genotoxic compounds. There are two major versions of this test, one in vivo and the other In vitro. The in vivo test normally uses mouse bone marrow or mouse peripheral blood. Micronuclei were first used to quantify...

, the genotoxicity of a chemical compound is tested.

See also

  • Macronucleus
    A macronucleus is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, the everyday tasks, such as metabolism...

  • Cancerous Micronuclei
    Cancerous Micronuclei
    - History :Theodor Boveri originally observed the fact that abnormal nuclear morphologies commonly occurred in cancer. Micronuclei are also referred to Howell-Jolly bodies; founded by hematologists William Henry Howell and Justin Marie Jolly in erythrocytes. Micronuclei induction by a chemical was...

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