Nucleus of Eukaryotic cells

It's discovered by Robert Brown in 1833, the nucleus is universal and prominent feature of all cells except the members of kingdom Monera. A few cells such as sieve tube elements of plants and red blood cells of many mammals may loose their nuclei when they become mature, but all cells have nuclei at some stage in their life. A cell typically contains a single nucleus. There are, however, a number of exception ( Vaucheria, Rhizopus, striated muscle cells, etc.) where more than one nucleus is present in a cell.



Shape and size of a nucleus:

The nucleus is generally spherical in shape, but it's shape is sometime related to the function and shape of the cell and then it may be ellipsoidal, fusiform, flattened or irregular. The nuclear margins are generally smooth but are sometimes lobulated as in leucocytes.

The location of the nucleus in the cell varies depending upon the species. In embryonic cells, the nucleus occupies almost the centre of the cell, but during differentiation, it is displaced and may be pushed against the cell membrane as the centre of the cell is often occupied by a large vacuoles.

The size of the nucleus varies in the cells of different organisms, in different cells of an organisms even in different stages of the same cell. The nucleus is usually larger in an active cell and smaller in a resting cell.


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Ultrastructure of an nucleus:

The structure of the nucleus can be best studied in interphase. It is surrounded by a nuclear envelope and contains chromatin, one or more nucleoli and the cytoplasm and provides a pathway for the transport of materials between the nucleus and the cytoplasm. The nuclear envelope is a double membrane structure; the outer membrane is continuous with the endoplasmic reticulum (ER). The nuclear envelope is perforated by nuclear pore that allow exchange of substances between the nucleus and the cytoplasm. There is a gel-like matrix within the nucleus called nucleoplasm, which contain chromatin and one or more nucleoli.


Function:

The nucleus is vitally important because it controls all metabolic activities of the cell by controlling the synthesis of enzymes. Chromosome present in the nucleus are responsible for carrying the genetic information from one cell generation to another. In addition, the nucleolus also takes part in the production of ribosomes, and protein synthesis.



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