The prominent spherical structure present within a living cell is the nucleus, which is controlling center of the cell. A typical eukaryotic nucleus is separated from cytoplasm by nuclear membrane. It is a double membrane structure possessing complicated nuclear pores, which provides direct connection between neucleus and cytoplasm.

The viscous liquid present within the nucleus is the nucleoplasm, which contains very much coiled chromatins threads that condense to appear as chromosomes during cell division and a spherical nucleolus. Chromatin threads are differentiated interest odark stained regions called heterochromatin and lightly stained regions called euchromatin an interphase nucleus. Nucleus possesses the chromosomes (DNA) in which all the informations are coded and are carried to cytoplasm by mRNA through the nuclear pores and decoded into proteins to control all the cellular activities. Nucleolus is the seat for biogenesis of RNA.