Nucleus is the centre of the cell also known as the controls centre or the brain of the cell. Nucleus and cytoplasm are separated by a thin membrane called nucleus membrane the fluid inside this is called nucloplast, A small spherical body inside nucleus is the Nucleolus it has several genetic Information of the cell like chromosomes and DNA etc. 
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Necleus is a small, spherical or oval structure located somewhere in the centre. It was discovered by Robert brown in 1831. The nucleus is composed of neclear membrane, necleoplasm, necleolus and chromatin network.