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| Nucleus |
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| Nucleus is the most conspicuous and the largest organelle in the cell. Most cells have a single nucleus and hence they are described as uninucleate. Some cells have two nuclei (binucleate) while a few may be even multinucleate. |
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| A nucleus generally occurs in all eukaryotic cells. However, a nucleus may be absent in some mature cells. Mammalian RBC and sieve tube cells in plants are enucleate (without a nucleus at maturity). |
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| The shape of the nucleus is highly variable. In most cases it is round. It may be oval or disc like or flat or lobed depending on the shape of the cell and its function. |
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| fig. 16.15- Nucleus in an Eukaryotic Cell |
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| Nucleus is bound by a double membrane, called nuclear membrane or nuclear envelope. It is discontinuous unlike the cell membrane and shows numerous pores called nuclear pores. Some of these pores are connected with branches of endoplasmic reticulum. The membrane encloses a clear fluid matrix called nucleoplasm or karyolymph. It encloses a fine network of thread-like structures, called chromatin network. It is composed of chromatin that contain DNA and histones arranged in a specific manner. The chromatin condenses to form distinct structures called chromosomes at the time of cell division. |
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| The nucleus also shows a round body called nucleolus in between the chromatin network. Nucleolus is rich in RNA and ribosomes, and as such take part in the synthesis of proteins. |
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| Nucleus is considered as the controlling centre of the cell. Substances from cytoplasm enter the nucleus freely and may influence the DNA and at the same time the DNA of the nucleus has its own influence on the activities inside the cytoplasm. |
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| The nucleus of a cell generally serves the following functions |
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 Nucleus maintains the cell and brings about its growth by directing the synthesis of structural proteins. |
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 Nucleus regulates the metabolic processes in the cell by directing the synthesis of functional proteins such as enzymes. |
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 Nucleus contains the genetic information necessary for reproduction, development and metabolism of the organism as a whole. |
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 Nucleus distributes the genetic material equally through the process of replication. |
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 Nucleus is involved in the formation of ribosomes. |
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| Although nucleus controls the cytoplasmic activities, it cannot exist on its own. Cytoplasm and nucleus of a cell are complementary to each other at the functional level. |
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