1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and neutrons, and contains nearly all of the mass of the atom while occupying only a tiny fraction of the volume. 2. [Cellular] A generally spherical structure encased in a double membrane that is found in most living eukaryotic cells. The nucleus contains hereditary information and directs the growth, metabolism, reproduction, and functioning of the cell.

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