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The nucleus is an organelle typically occupying about a tenth of the eukaryotic cell volume. It has a central role in gene expression, heredity and cell division by virtue of the presence of DNA. Indeed, the nucleus theoretically acts as a haven for precise interpretation of the genetic code away from the myriad of cytoplasmic functions.
There are a variety of proteins present which aid function e.g.:
DNA may be stored in a variety of ways within the nucleus e.g. within chromosomes, chromatids, chromatin, nucleosomes, Barr body or nucleolus. The nucleolus has a special function as the origin of ribosomal RNA. All nuclear components are suspended in amorphous nucleoplasm - karyolymph - and enveloped by a specialised nuclear membrane.
A few cells have dispensed with the nucleus at the expense of function e.g. haemoglobin storage in the mature erythrocyte.