The central core, structure, or focal point of an object. The nucleus of a living cell is a roughly spherical unit at the centre of the cell. It contains the chromosomes (composed mainly of nucleic acid), which are responsible for directing the cell’s activities, and is surrounded by a membrane. The membrane has small pores through which various substances can pass between the nucleus and the cytoplasm, a thick fluid that forms the bulk of the cell. Usually, the nucleus has 1 nucleolus, a smaller dense region with no membrane that is concerned with protein manufacture. A nerve nucleus is a group of neurons (nerve cells) within the brain and spinal cord that work together to perform a particular function. The nucleus of an atom, composed of protons and neutrons, accounts for almost the total mass of the atom but only a tiny proportion of its volume. Nuclear energy is produced through changes in atomic nuclei.


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