Nuclei were first described by Brown in 1833 and were quickly recognized as a constant feature of animal and plant cells. Typically they are spherical or ovoid bodies, but other shapes are not uncommon; for example, polymorphonuclear leucocytes have a lobulose nucleus of several interconnected parts, ciliate protozoans are characterized by a kidney-shaped macronucleus, and there are various shapes among the lower eukaryotes. Some nuclei, like those of the silk glands of the silk-worm, have finger-like extensions that greatly increase their surface area. In many cases, neither the biological significance of nuclear shape nor the factors causing it have been defined, although these are interesting cell biological Problems. Nuclei vary in size from about 3 µm to 25 µm in diameter, depending on cell type, and contain chromosome diploid numbers that range from 6 (Indian muntjac) to 92 (Anotomys leander, a rodent) for mammalian species. Some polyploid species of plants and invertebrates have several hundred chromosomes. A direct relationship between chromosome ploidy and nuclear size is found in some species, e.g. the sea urchin, but not in others. In fact, there are many examples that show that nuclear size is not obligatorily linked with chromosome number or DNA content: the nuclei in different tissues of the same mammal vary considerably in size, despite having the same chromosome number. One or more nucleoli are present in the nuclei of most active tissues. These are conspicuous granular bodies without membrane, typically associated with specific chromosomes. They are the sites of ribosome assembly and are dynamic structures that change in size, disaggregating during mitosis and reforming in the daughter cells.
KeywordsNuclear Envelope Silk Gland Chromatin Fibre Indian Muntjac Maturation Promoter Factor
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