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The term micronucleus may also refer to the smaller nucleus in ciliate protozoans, such as the Paramecium. In mitosis it divides by fission , and in conjugation a pair of gamete micronuclei undergo reciprocal fusion to form a zygote nucleus, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission.
Ciliates have two kinds of nuclei in a single cell, a somatic macronucleus and a germline micronucleus. [78] In humans, skeletal muscle cells , also called myocytes and syncytium , become multinucleated during development; the resulting arrangement of nuclei near the periphery of the cells allows maximal intracellular space for myofibrils . [ 1 ]
Micronucleus induction by a chemical was first reported in Ehrlich ascites tumor cells treated with colchicine. The effect of environmental stressors on the expression of micronuclei was first analyzed in root tip cells under ionizing radiation. It can be inferred that nuclear abnormalities are a result of various molecular mechanisms.
The micronucleus is the generative, or germline nucleus, containing the genetic material that is passed along from one generation to the next. [ 31 ] Paramecium reproduction is asexual , by binary fission , which has been characterized as "the sole mode of reproduction in ciliates" ( conjugation being a sexual phenomenon, not directly resulting ...
During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus.
The micronucleus genome contains five chromosomes that undergo mitosis during micronuclear division and meiosis during conjugation, which is the sexual division of the micronucleus. The macronuclear genome is broken down and catabolized once per life cycle during conjugation, allowing it to be site-specific, and a new macronucleus ...
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A micronucleus test is a test used in toxicological screening for potential genotoxic compounds. The assay is now recognized as one of the most successful and reliable assays for genotoxic carcinogens , i.e., carcinogens that act by causing genetic damage and is recommended by the OECD guideline for the testing of chemicals. [ 1 ]