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A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugation, the macronucleus disintegrates, and a new one is formed by karyogamy of the micronuclei.
The micronucleus has chromatin that is densely packed as well as an absence of nucleoli. [4] The micronucleus forms zygotic nuclei during meiosis during conjugation. These zygotic nuclei can follow a process and differentiate into macronucleus or micronucleus cells. Macronucleus cells, on the other hand, differentiate by changes to the DNA.
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.
The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the small nuclear RNA for vegetative growth. [16] [15] Division of the macronucleus occurs in most ciliate species, apart from those in class Karyorelictea, whose macronuclei are replaced every time the cell divides. [17]
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]
The macronucleus divides via amitosis, whereas the micronucleus undergoes typical mitosis. During sexual development a new macronucleus is formed from the meiosis of the micronucleus, where the removal of transposons occurs. [1] On the division or reproduction of ciliates, the two nuclei are under several epigenetic controls.
The final macronucleus has greater genetic content than micronucleus. In ciliates there are two types of diminution: the first is fragmentation of genome and loss of repetitive sequences and the second is deletion of internally eliminated sequences in chromosomes and the rejoining of remaining DNA fragments.
In trophozoites, the two nuclei are visible. The macronucleus is long and sausage-shaped, and the spherical micronucleus is nested next to it, often hidden by the macronucleus. The opening, known as the peristome, at the pointed anterior end leads to the cytostome, or the mouth. Cysts are smaller than trophozoites and are round and have a tough ...