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Pronunciation: nu'kle-us, nu'kle-i


  1. In cytology, typically a rounded or oval mass of protoplasm within the cytoplasm of a plant or animal cell; it is surrounded by a nuclear envelope that encloses euchromatin, heterochromatin, and one or more nucleoli, and undergoes mitosis during cell division.
  2. By extension, because of similar function, the genome of microorganisms (microbes), which is relatively simple in structure, lacks a nuclear envelope or membrane and does not undergo mitosis during replication.
  3. In neuroanatomy, a group of nerve cell bodies in the brain or spinal cord that can be demarcated from neighboring groups on the basis of either differences in cell type or the presence of a surrounding zone of nerve fibers or cell-poor neuropil.
  4. Any substance (foreign body, mucus, crystal) around which a urinary or other calculus has formed.
  5. The central portion of an atom (composed of protons and neutrons) where most mass and all the positive charge are concentrated.
  6. A particle on which a crystal, droplet, or bubble forms.
  7. A characteristic arrangement of atoms in a series of molecules; the benzene nucleus is a series of aromatic compounds.

Synonym(s): karyon

[L. a little nut, the kernel, stone of fruits, the inside of a thing, dim. of nux, nut]

See Also: virion

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