The nucleus is the most significant membrane enclosed part of the eukaryotic cells which contains chromosomes and controls all the activities of the cell. The nucleus is the most well-known organelle as compared to other cell organelles, which accounts for about ten percent of the cell's volume. Generally, a eukaryotic cell has only one nucleus. But some eukaryotic cells are enucleate cells i.e., without nucleus, for example, red blood cells (RBCs); whereas, some are multinucleate and consists of two or more nuclei, for example, slime molds.
All eukaryotic cells except mature mammalian RBC and sievetubes in phloem of plants contain nucleus. Generally, it is situated at the centre of the cells but in a few cases, it may occupy a peripheral position. For example-in mature plant cells, with increase of size of the vacuole, the nucleus along with the cytoplasm is pushed peripherally ; in adipose tissue of animals, the nucleus is also pushed toward one end of the cell by the accumulating mass of fat; the nucleus of skeletal muscle cells are also located just beneath the cell membrane.
In general, each cell contains a single nucleus but in certain animal cells such as, Paramoecium, cartilage cells, cardiac muscle cells etc. and plants cells such as, alga like Vaucheria, fungus like Rhizopm, cells of latex vessels etc., more than one nuclei are present. Multinucleated animal cells are called syncitial cells, whereas such plant cells are called coenocytes.
Nuclei initiate from the pre-existing ones and are never formed afresh. During cell division, the nucleus of the mother cell divides to form the daughter Nucleus.
The nucleus is the largest of all cellular organelles. Its’ having diameter 5-25 μm. The size of the nucleus is comparative to the chromosome number. Thus, in haploid cells, the size of the nucleus is smaller than that of the diploid cells. In each somatic cell, during the inter-phase, the nucleus attains a specific size that depends on the DNA and protein content and also on the efficient activity of the nucleus.
Depending upon the number, size and nature of nucleus, the cells may be of the following kinds: -
i) Eukaryon: - It is a cell which contains a single nucleus.
ii) Dikaryon: - It is a cell containing two nuclei of the same kind.
iii) Polykaryon: - It is a cell that contains a number of nuclei of the same type.
iv) Heterokaryon: - It is single cells which contain nuclei of two types, produced by fusion of two cells.
v) Synkaryon: - It is a hybrid cell line generated by a heterokaryon whose nuclei have entered mitosis synchronously and merged.
Depending on its nature the nucleus itself may be of the following types: -
i) Amphinucleus: - It is a diploid type of nucleus and it contains two sets of genome. In somatic cells it is found.
ii) Heminucleus: - It is a haploid type of nucleus an it contains only one set of genome. Heminucleus type of nucleus is found in gametes.
iii) Pronucleus: - The haploid nucleus of an ovum or spermatozoon, present in a fertilized ovum before their fusion, is called pronucleus. The nucleus of an ovum before it unites with that of the spermatozoon which has penetrated it is called female pronucleus. The nucleus of the spermatozoon after it has penetrated an ovum is called male pronucleus.
iv) Micronucleus and Macronucleus: - In a dikaryon, the two nuclei are unequal in size. Among these, the smaller one is called micronucleus and the bigger one is called macronucleus.
Cell nucleus is the most vital centre of the cell and it regulates all the behavior of the cell. Hence, the nucleus is measured as the cell brain. It plays an important role in the production of RNA, ribosome and protein and it takes a major part in cell division. Due to the presence of DNA in it, the cell nucleus functions as the intermediary of heredity.
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