From Biology-Online Dictionary | Biology-Online Dictionary
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In general, a nucleus is the core or the central part around which other parts are grouped or gathered.

In Biology, a nucleus is the large, membrane-bounded organelle that contains the genetic material, in the form of multiple linear DNA molecules organized into structures called chromosomes. Its major functions are to maintain the integrity of DNA and to control cellular activities such as metabolism, growth, and reproduction by regulating gene expression. It has three main components: the nucleolus, the chromatin and the nuclear envelope. The nuclear envelope has nuclear pores to control the movement of molecules between the nucleoplasm and the cytoplasm.

In other fields, such as:

In Physics, a nucleus refers to the positively-charged center of an atom that usually contains the protons and neutrons.

In Chemistry, a nucleus is a fundamental arrangement of atoms that occur in compounds through substitution of atoms without a change in structure.

In Astronomy, a nucleus is the center of the head of a comet or the central or brightest part of a nebula or galaxy.

In Meteorology, a nucleus is a particle on which water vapor molecules accumulate in free air to form water drops or ice crystals.

In Botany, a nucleus is the central kernel of a nut or seed, or the center of a starch granule.

In Anatomy, a nucleus is a group of specialized nerve cells in the brain or spinal cord.